Shame on Trust in Distributed Systems
Shame on Trust in Distributed Systems
Trent Jaeger,Patrick McDaniel,Luke St.Clair Pennsylvania State University
Ram′o n C′a ceres,Reiner Sailer IBM T.J.Watson Research Center
1Introduction
Approaches for building secure,distributed systems have fundamental limitations that prevent the construction of dy-namic,Internet-scale systems.In this paper,we propose a concept of a shared reference monitor or Shamon that we believe will provide a basis for overcoming these limita-tions.First,distributed systems lack a principled basis for trust in the trusted computing bases of member machines. In most distributed systems,a trusted computing base is as-sumed.However,the fear of compromise due to miscon-?guration or vulnerable software limits the cases where this assumption can be applied in practice.Where such trust is not assumed,current solutions are not scalable to large systems[7,20].Second,current systems do not ensure the enforcement of the?exible,distributed system secu-rity goals.Mandatory access control(MAC)policies aim to describe enforceable security goals,but?exible MAC solutions,such as SELinux,do not even provide a scal-able solution for a single machine(due to the complexity of UNIX systems),much less a distributed system.A signi?-cant change in approach is necessary to develop a principled trusted computing base that enforces system security goals and scales to large distributed systems.
Our proposal is to develop scalable mechanisms for com-posing a veri?able reference monitoring infrastructure that spans Internet-wide distributed systems.We refer to a set of reference monitors that provides coherent security guarantees across multiple physical machines as a Sha-mon1.While this may sound like a mere extension of the well-known reference monitor concept,we propose sev-eral key differences:(1)the credentials of secure hardware (e.g.,Trusted Computing Group’s Trusted Platform Mod-ule),rather than users,are used to authenticate individual reference monitoring systems in the Shamon;(2)trust in the Shamon is based on attestation of reference monitoring properties:tamperproo?ng,mediation,and simplicity of design;(3)virtual machine monitoring is used to establish coarse-grained domains,which results in signi?cant sim-pli?cation of MAC policies;(4)policy analyses verify that these MAC policies satisfy the Shamon application’s secu-rity goals when enforced by the Shamon;and(5)based on this restricted de?nition of trust,a focused logic is de?ned that enables scalable evaluation of this trust by components 1The name is short for Shared Monitor and related to the word shaman meaning“...a medium...who practices...control over natural events”words removed for effect,not necessarily accuracy).
of the distributed system that is also resilient to dynamic changes in the application.
The Shamon approach addresses the fundamental chal-lenges described above.First,trust is built from the bottom-up via secure hardware credentials that enable attestations of virtual machine-based enforcement for each machine. Second,the MAC policy enforced by the Shamon is used to prove enforcement of system security goals.We de-?ne a logical representation for verifying these criteria that enables scalable management of large Shamon even under changes in application con?guration.Each of the?ve tasks that convert a reference monitor into a Shamon presents sub-stantial research challenges,but we aim to demonstrate that each has tractable solution potential and that the resultant Shamon system will provide a foundation for large-scale distributed authorization.To motivate its design,we intro-duce our prototype application of the Shamon in the follow-ing section.
2Application
The Playpen is a Xen-based,virtual machine(VM)environ-ment for the students taking security courses at Pennsylva-nia State University.Each student is given their own virtual machines in the Playpen.Over the course of the semester, students are required to con?gure and build security appara-tus to defend their machines against attacks from the faculty and TAs.The isolation,persistence,and mobility of the VM environment provides ideal conditions for pedagogy:users can experiment with security apparatus under the controlled environment.
The current Playpen is the prototype for a larger project supporting wide-area mobile and secure computing envi-ronments.The long term goal is to extend the Playpen to en-compass all aspects of university life.In this,a user would be given one or more virtual machines that would migrate to the location where they are working.The central chal-lenge of this work is to support the users’ability to move freely within the university environment.The system must securely support arbitrary migration to previously unknown hardware at a previously unknown location and share data with previously unknown collaborators.Note that while the environment aims at a single university system,we are not centrally-administered:there is different administration at each campus,and some departments also administer their own machines.
Consider a typical day of Alice the graduate student in 1
this new university.She wakes up at noon and goes to class. Alice joins a live coalition of class participants by logging into a host in her classroom.She exits the coalition at the end of class,and at lunch she surfs the Internet and ex-changes personal communication within her protected en-vironment at the local student union.After lunch,she heads to the laboratory and performs research and shares data with the other graduate students.At the end of the day,she meets with her advisor and shares summary data and exchanges results.She heads home and plays a massively multiplayer game with thousands of other gamers until dawn over the Internet.
Such is the nature of university life.The”roles”of Alice’s computing environment and the environments in which she interacts evolve constantly;from class partic-ipant,personal communication,researcher,advisee,and gamer.Moreover,the set of hosts to which she has an as-sociation is also changing.What is interesting here is not that this somehow changes the way Alice lives,but that her computing environment follows her throughout her life. The security challenges of this environment are non-trivial.The physical machines within the open university environment are largely unknown and often compromised.2 The applications are as diverse as the environments in which Alice lives,from classroom to research to gaming.Further-more,the collaborations in which Alice participates change hourly,and often form and disband organically.It is clear that:(1)supporting this environment requires signi?cant se-curity,and(2)current commodity environments(e.g.,dis-tributed?le systems and VPNs)do not support it.Note that large corporate environments are similar–users will move freely through a largely insecure complex and use data and applications as needed.
Research that enables articulation of?ner-grained poli-cies across distributed systems,for distributed?le access (e.g.,[15,3])and trust management(e.g.,[4,14]),often assume trust in the trusted computing base as well.An ex-ception is the Taos operating system approach[2]which has a form of secure booting for establishing trust in the in-frastructure.However,building trust in a single machine is insuf?cient.We need to build trust in enforcement across distributed applications within the distributed system(e.g., each of Alice’s roles)and ensure that distributed authoriza-tion policy enforces the security goals of those applications. In order for this to be truly useful,it must enable large dis-tributed applications to be supported.
The?ve key design requirements identi?ed in the pre-ceding section are re?ected in the university environment: users need to vet the many untrusted machines in some re-liable and secure way;they need to vet the policy enforce-ment infrastructure(simplicity,tamperproo?ng,and media-tion);they need to articulate an inter-host(sharing)security policy;they need to ensure that all hosts sharing data pro-2Would any sane person completely trust a host in a open university laboratory?Seriously.vide a consistent view of security;and it must scale–there are over41,000students at Penn State spread out over24 campuses.
3Coalitions and Shamon
Figure1:Example of a distributed coalition.Virtual ma-chine(VM)instances sharing common Mandatory Access Con-trol(MAC)labels on multiple physical hypervisor systems are all members of the same coalition.
Figure1illustrates the conceptual idea for future dis-tributed applications.A distributed application is a coali-tion of VMs that executes across multiple physical plat-forms.Each member of the coalition may reside on a dif-ferent physical machine,which may itself execute multiple coalitions.The physical machines themselves each have a reference monitor capable of enforcing MAC policies over all of their VMs.
We de?ne the Shamon as follows.A Shamon is a set of reference monitors serving a coalition by enforcing its se-curity goals.A reference monitor may belong to multiple Shamon,so its enforcement must ensure the satisfaction of the security goals for each.The challenge is to establish trust in the Shamon reference monitors’enforcement of a coalition security goal.This trust must be upheld in a scal-able fashion as VMs join the coalition or migrate between machines.In so doing,the Shamon provides authorization across an entire coalition as if it were a single machine. For Alice,each VM in a coalition represents an instance of her work on a speci?c task.She may work on her re-search in the lab,in class,or in the student union,and her re-search coalition enables these VMs to communicate.How-ever,her gaming and browsing VMs would not be part of this coalition.In fact,the research coalition enables iso-lation of the research VMs from the gaming and browsing VMs even when they are running on the same machine at the same time.
We envision different requirements for managing Alice VMs than in a traditional VM isolation system.First,the se-curity focus is to separate Alice’s workloads based on trust (i.e.,trusted from untrusted)or domain(i.e.,research from school work),but total isolation is too restrictive.For ex-
ample,some data from an untrusted domain(e.g.,Google search results)may be useful in a trusted domain(e.g.,re-search paper).These are?ner-grained and more?exible security requirements than are typical of VM systems.Sec-ond,the VMs will be more dynamic and composed into larger systems than is traditionally the case.Also,some VMs may be destroyed on a frequent basis.In addition, large-scale coalitions with changing memberships may be constructed for particular causes that Alice may join,such as conferences,auctions,rallies,or social events.
4Shamon Challenges
The basic mechanism for composing a coalition is shown in Figure2.Each machine that will join a coalition must have a credential registered with the coalition authority,such that statements made on behalf of the machine(e.g.,attesta-tions)can be veri?ed(messages1and2).Joining requires attestation of Shamon properties which results in a proof of acceptance from the authority(messages3and4).This proof is used to communicate with another coalition mem-ber,and this coalition member establishes a dependency on this proof and any status changes from the authority(mes-sages5through7).
The scalability of the approach comes from reusing coali-tion authority attestations at join time and deferring proof of integrity until communication is initiated.These advantages are similar to typical PKI approaches where the proof of the possession of a private key is generated by an authority and veri?cation of this possession is done when communication is initiated.
There are some important differences,however,between this approach and PKI.First,a major bene?t is that trust is built from machines rather than individuals.Thus,trust in the trusted computing base is built in a bottom-up manner along with the booting of the trusted computing base itself. Keys can be stored and used in secure hardware rather than in application software.Second,a major challenge is that a reference monitor’s status may change,motivating a revoca-tion of the member.Remember that we only depend on the reference monitor and coalition MAC policy being correct. Normally,these will not change,but we need a lightweight mechanism to convey the status quo without missing a com-promise.In theory,TPM statements of the integrity value (and a nonce for freshness)could be provided and checked frequently at a low cost,except the current TPMs are slow and use public key cryptography.The bene?t of bottom-up mechanism to establish trust should motivate an investiga-tion into making ef?cient integrity maintenance practical. Below,we assess the?ve Shamon features from Section1 relative to Alice’s requirements.
Shamon Authentication In order to verify a Shamon for joining a coalition,we must be able to authenticate the ma-chine upon which the Shamon runs.Secure hardware of the machine,such as the Trusted Computing Group’s Trusted Platform Module[1](TPM)is capable of generating cre-dentials that can be certi?ed by an authority(e.g.,using Di-rect Anonymous Attestation).Such credentials can be used to register the machine for use in a coalition via such an au-thority,called a coalition authority,as shown in Figure2. Note that since TPMs are not tamperproof,some degree of physical security is required.For Alice,different physical requirements may be necessary for different coalitions:the research coalition may require machines protected by the university,whereas her coalition for completing her tax re-turn may only require machines that meet her physical secu-rity requirements.Acceptable models combining physical security and credentials are a research challenge. Shamon Attestation When Alice picks a machine to join a particular coalition,this machine must prove that it can join the Shamon.This involves the following steps:(1)pro-vide an integrity measurement of Shamon components us-ing remote attestation protocols based on the TPM;(2)ob-tain the coalition’s MAC policy from a coalition authority; and(3)construct a proof of its consistency with this policy (e.g.,in labeling)and its ability to enforce the security goals required.Remote attestation approaches have been devel-oped that enable measurement of trusted code and informa-tion?ow policies for trusted applications[11].We envision a signi?cant bene?t from having the coalition determine the MAC policy for the application rather than trying to con-?gure systems a priori.Note that we can piggy-back the attestation veri?cation on the negotiation of secure commu-nication channels(done as a result of message5),such as for Labeled IPsec[10].
User Authentication Also,the users must authenticate themselves to the Shamon infrastructure.The challenge is that the user does not necessarily have trust in the physi-cal platform that she is using at a particular time.Even a machine that may appear to be shutdown may actually be compromised(e.g.,by a virtual machine rootkit[13]).A challenge for the user is to establish that she can submit her authentication secrets without fear of losing them.The user must be to verify a statement from an authority she trusts (e.g.,a coalition authority)that vouches for the fresh attes-tation of the platform that she is actually using.Enabling a user to verify the authenticity of a machine requires a trusted path in general,although some social mechanisms may be effective in controlled environments,such as a cam-pus(e.g.,trusted labeling,such as proposed for room ac-cess[17]).
Shamon MAC Policy Simplicity The basis for simplify-ing MAC policy is the use of virtual machine communica-tion as the basis for security guarantees.In a Xen-based sys-tem,sHype[19]controls inter-VM communication by au-thorizing only Xen grant tables(i.e.,shared memory),Xen event channels(i.e.,basic IPC),and Linux IPsec tunnels (i.e.,network communication via Xen’s domain0)must be controlled.Other system resources,such as disk space and memory are partitioned for virtual machines,so they are
1:Register physical machine(e.g.,TPM)with
veri?cation infrastructure
2:Con?rm successful registration
3:Join coalition request(attest enforcement
and security goals)
4:Proof of veri?cation for join
5:Alice sends request(IPsec negotiation)
6:Accept proof of veri?cation
7:Notify change to member status
Figure2:Coalition member join process:Secure hardware enables veri?cation of Shamon enforcement of security goals which is conveyed when Alice participates with another member.
isolated by default.As a result,a much simpler MAC pol-
icy for inter-VM communication can be de?ned[16]than
for a UNIX system(e.g.,SELinux strict policy).
VM isolation is likely too restrictive for Alice’s tasks,
as we discussed earlier,because there will be a bene?t in
transferring information from applications in one VM to
another.Because VMs will be application-focused,we sur-
mise that it will be more appropriate for applications to dic-
tate the enforcement of these security requirements rather
than operating systems(although they may be implemented
via OS mechanisms).Recent work in information-?ow lan-
guages[9]and security-aware code[6]may provide a ba-
sis for integrity protections based on a combination of VM
protections and limited application protections[21].For ex-
ample,Alice’s document application may prove that it can
handle the receipt of low integrity text data via limited,?l-
tering interfaces.
Shamon Security Goal Veri?cation A consistent secu-
rity policy includes all facets of MAC policy de?nition,in-
cluding subjects and object labeling mechanisms(i.e.,de-
ciding how labels are assigned),permission assignments
(i.e.,assigning permissions to subjects),and label transi-
tions(i.e.,changes in subject labels for a process),if any.
Since this is provided to the Shamon upon joining a coali-
tion,the MAC policies used in the coalition may be mea-
sured via attestation.Remote parties can then verify that the
MAC policy satis?es coalition security goals in the context
of the Shamon and other coalitions running on that system.
Recent work in MAC policy analysis shows that informa-
tion?ow security properties can be veri?ed for very com-
plex policies[8,12,22].In general,veri?cation involves
detection of information?ow problems where secret data
may be leaked or low integrity data may enter a trusted ap-
plication.Thus,veri?cation can detect a VM that is leaking
Alice’s research information or receiving an untrusted exe-
https://www.wendangku.net/doc/b614956955.html,ing application-level enforcement approaches as
described above would enable limited I/O for applications
entrusted to perform such operations using only approved
interfaces.
Shamon Trust Logic In order to reason about the trust
state of the Shamon across large system,we need a rep-
resentation for this state.We propose a basic predicate
logic(space limits prevent its discussion)that de?nes a few
speci?c predicates that enable reasoning about attestations
meeting enforcement goals,MAC policies meeting security
goals,and virtual machines running on enforcing machines.
We believe that a logic focused on these speci?c types of
properties will enable practical system constructions,but
much research is to be done.The challenges in designing
such a logic include:(1)accuracy in representing real be-
havior;(2)ability to ef?ciently handle coalition dynamics
(i.e.,use monotonic logic under changing conditions);and
(3)limitation of manual policy speci?cation.Alice’s ma-
chine must be able to correctly determine that no attack on
the enforcement of coalition security goals(i.e.,the Sha-
mon)can go undetected(within a reasonable probability),
even when the coalition membership changes(perhaps due
to a detected attack),and with little or no input from Alice
or her system’s administrators.
5Thinking About Usability
To illustrate the challenges in building a distributed sys-
tem that can be practical,we examine two approaches used
to build distributed systems that have had some success:
(1)automated teller machine(ATM)networks and(2)vir-
tual network computing(VNC).We do not mean to imply
that coalitions supported by Shamon have the same require-
ments as ATMs or VNCs(although close to some versions
of the latter),but reviewing the requirements of a success-
ful distributed infrastructure may be illuminating in trying
to build a more?exible approach.We then compare the
environment of Shamon systems to that of ATM networks
and VNC systems to identify challenges in making the Sha-
mon approach practical.
5.1Automated Teller Machine Networks
ATMs enable users to perform speci?c types of transac-
tions using a distributed network of machines.Like the
Shamon approach,the ATM networks de?ne approaches
for authentication(users and ATMs),attestation(banks ver-
ify ATM software),and security goals(speci?c to banking
transactions).
While there have been rogue ATMs installed in the past
that have been used to steal user’s PINs,users have come to
trust a visual authentication of ATMs.Such authentication
partly comes from physical security(i.e.,located in a bank or public place)where someone might notice a rogue sys-tem.Of course,the fact that the banks assume the responsi-bility for most fraud is also important.In the Shamon sys-tem,we may want to leverage social notions of location and responsibility where possible.For example,a university lab may guarantee secure boot of its machines.However,more effort will be required in ensuring this guarantee than an ATM case.Otherwise,users will have to authenticate ma-chines via coalition authorities using a strong cryptographic https://www.wendangku.net/doc/b614956955.html,ers cannot enter secrets(e.g.,ATM PINs)into machines unless a means for validating their authenticity can be provided.
The banks require attestation of the ATM systems before performing transactions on behalf of customers.Currently, attestation is done by the tamperproof,secure coprocessor. The cost of tamperproo?ng is too high for Shamon systems, so we can only assume Trusted Computing Group(TCG) Trusted Platform Modules(TPMs)or similar.The lack of tamperproo?ng means that physical security guarantees are required for security.Perhaps the agreements for physical security necessary to authenticate the coalition systems can be leveraged.
The most signi?cant difference between an ATM and a Shamon system will be that the user can get their applica-tions downloaded to the coalition machine and request en-forcement of their security requirements by that machine. ATMs provide applications for the speci?c authenticated user,and no sharing of information between applications is permitted.A user may have multiple application VMs and must be able to share information among them.Also, these VMs may have to interact with other VMs on other machines.The coalition authority must support the user in identifying applications,security requirements,and ensur-ing that the security requirements are enforced.The individ-ual machines must be able to interpret and enforce the secu-rity requirements.The expanded application suite probably has a bigger impact on users than coalition authorities or machines.Well-de?ned,but effective,means of usage will have to be determined.
5.2Virtual Network Computing
In Virtual Network Computing(VNC),the interface to a particular base machine is exported to another remote ma-chine where the user can interact with the base machine in the same manner as if the user was local to that machine. Such a mechanism has evolved from the export of individ-ual terminals via XWindows[23]to the export of entire screens to machines with different operating systems via RealVNC[18]to the export of interfaces via HTTP to any machine via https://www.wendangku.net/doc/b614956955.html,[5].The latter enables access to a base machine via a web browser on any remote machine with minimal previous con?guration.
Security on these systems mainly focuses on user authen-tication and secure communication.RealVNC aims to pro-vide a remote access,but the idea is that the user has a re-mote machine that is under her control(e.g.,her personal laptop).Thus,attestation is not strictly https://www.wendangku.net/doc/b614956955.html, advertises access to your computer from any machine on the Internet,but security focuses on user au-thentication and secure communication rather than the in-tegrity of the computer being used.The mobile user goes to the https://www.wendangku.net/doc/b614956955.html, website to be authenticated and setup a secure communication channel to the base machine.The website authenticates the user,but does not verify the in-tegrity remote system.It is the user’s responsibility to de-tect a malicious system much as in the ATM case,but a fraudulent or compromised computer system is much more likely in this case.Shamon attestation aims to verify the in-tegrity of such machines to ensure mandatory integrity re-quirements are met.
In VNC systems,access control requirements are en-forced on the base machine as that is where the compu-tation takes place.Thus,VNCs export interfaces,but not access control requirements.As a result,there is no con-trol of information once it reaches the remote machine.A compromised machine can leak data,so mandatory access control(e.g.,multi-level security)cannot be enforced.The Shamon approach aims to ensure coherent security enforce-ment across the machines in each coalition.
6Conclusions
In this paper,we proposed the concept of a shared ref-erence monitor or Shamon as the basis for building dis-tributed systems that enforce intended security goals.This approach leverages the secure hardware for building trust from the bottom-up,virtual machines for simplifying MAC policy for veri?cation,and a trust logic for representing Shamon trust state.Initial work shows promise,but key challenges remain,such as accurately tracking the integrity of individual systems.
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https://www.wendangku.net/doc/b614956955.html,/.
英语介词口诀
英语介词用法口诀 早、午、晚要用in，at黎明、午夜、点与分。 年、月、年月、季节、周，阳光、灯、影、衣、冒in。 将来时态in...以后，小处at大处in。 有形with无形by，语言、单位、材料in。 特征、方面与方式，心情成语惯用in。 介词at和to表方向，攻击、位置、恶、善分。 日子、日期、年月日，星期加上早、午、晚， 收音、农场、值日on，关于、基础、靠、著论。 着、罢、出售、偷、公、假，故意、支付、相反，准。 特定时日和"一……就"，on后常接动名词。 年、月、日加早、午、晚，of之前on代in。 步行、驴、马、玩笑on，cab，carriage则用in。 at山脚、门口、在当前，速、温、日落、价、核心。 工具、和、同随with，具有、独立、就、原因。 就……来说宾译主，对、有、方状、表细分。 海、陆、空、车、偶、被by，单数、人类know to man。 this、that、tomorrow，yesterday，next、last、one。 接年、月、季、星期、周，介词省略已习惯。 over、under正上下，above、below则不然， 若与数量词连用，混合使用亦无关。' beyond超出、无、不能，against靠着，对与反。 besides，except分内外，among之内along沿。 同类比较except，加for异类记心间。 原状because of,、owing to、due to表语形容词 under后接修、建中，of、from物、化分。 before、after表一点, ago、later表一段。 before能接完成时，ago过去极有限。 since以来during间，since时态多变换。 与之相比beside，除了last but one。 复不定for、找、价、原，对、给、段、去、为、作、赞。 快到、对、向towards，工、学、军、城、北、上、南。 but for否定用虚拟，复合介词待后言。 ing型由于鉴，除了除外与包合。 之后、关于、在......方面，有关介词须记全。 in内to外表位置，山、水、国界to在前。
英语介词用法大全
英语介词用法大全 TTA standardization office【TTA 5AB- TTAK 08- TTA 2C】
介词（The Preposition）又叫做前置词，通常置于名词之前。它是一种虚词，不需要重读，在句中不单独作任何句子成分，只表示其后的名词或相当于名词的词语与其他句子成分的关系。中国学生在使用英语进行书面或口头表达时，往往会出现遗漏介词或误用介词的错误，因此各类考试语法的结构部分均有这方面的测试内容。 1. 介词的种类 英语中最常用的介词，按照不同的分类标准可分为以下几类： (1). 简单介词、复合介词和短语介词 ①.简单介词是指单一介词。如： at , in ,of ,by , about , for, from , except , since, near, with 等。②. 复合介词是指由两个简单介词组成的介词。如： Inside, outside , onto, into , throughout, without , as to as for , unpon, except for 等。 ③. 短语介词是指由短语构成的介词。如： In front of , by means o f, on behalf of, in spite of , by way of , in favor of , in regard to 等。 (2). 按词义分类 {1} 表地点（包括动向）的介词。如： About ,above, across, after, along , among, around , at, before, behind, below, beneath, beside, between , beyond ,by, down, from, in, into , near, off, on, over, through, throught, to, towards,, under, up, unpon, with, within , without 等。 {2} 表时间的介词。如： About, after, around , as , at, before , behind , between , by, during, for, from, in, into, of, on, over, past, since, through, throughout, till(until) , to, towards , within 等。 {3} 表除去的介词。如： beside , but, except等。 {4} 表比较的介词。如： As, like, above, over等。 {5} 表反对的介词。如： againt ,with 等。 {6} 表原因、目的的介词。如： for, with, from 等。 {7} 表结果的介词。如： to, with , without 等。 {8} 表手段、方式的介词。如： by, in ,with 等。 {9} 表所属的介词。如： of , with 等。 {10} 表条件的介词。如：
表示地点的介词
表示地点的介词 in、on、behind、next to、near、over、under (1) in在……里面：The pencil is in the desk. 铅笔在课桌里。 (2) on在……上面：There are some apple on the tree. 树上有些苹果。 (3) under在……下面/正下方：What's under your desk? 你书桌底下是什么？ (4) over在……正上方：There is a shelf over the table. 桌子上方有一个书架。 (5) above 在……斜上方：Raise your arms above your head. (6) below 在……斜下方：Her skirt came below her knees. (7) behind在……之后：There is a bike behind the tree. 树后有一辆自行车。 (8) next to在……旁边：There is a shop next to the school. 理发店隔壁是一家咖啡馆。 (9) near在……附近：My bed is near the window. 我的床在窗户旁。 (10) by在……旁：He was sitting by the window. (11) beside 在旁边He sits down beside Jim. 一组：over, above和on的用法 1) over指在…的正上方，表示垂直在上。如 There is a lamp over the desk. 2) above指在上方，属于斜上方。如： Raise your arms above your head. 3) on指在上面，表示两物体接触。如： There is a cup on the table. 二组：under / below的用法： 1) under在……下面/正下方： What's under your desk? 2) below 在……斜下方： Her skirt came below her knees 三组：in 和on表示“在……上” 1)门/窗一类——镶嵌在墙里的，用in; 2)字画一类——挂/贴在墙面上的，用on ( ) 1 He put up a map ___ the back wall because there was a hole ______ it. A. on; on B. at; in C. on; in D. on; at ( ) 2 There is a door___ the wall. A. on B. to C. of D.in 3)人/鸟其他东西在树上，用in; 4)枝叶果实长在树上的，用on ( ) 1 There are some birds singing___ the trees. A. in B. on C. at D. from ( ) 2 There are so many apples___ that tree. A.in B, on C of D.from
介词in,on.at,for.with,by,of的基本用法
介词用法知多少 介词是英语中最活跃的词类之一。同一个汉语词汇在英语中可译成不同的英语介词。例如汉语中的“用”可译成：（1）用英语（in English）；（2）用小刀（with a knife）；（3）用手工（by hand）；（4）用墨水（in ink）等。所以，千万不要以为记住介词的一两种意思就掌握了这个介词的用法，其实介词的用法非常广泛，搭配能力很强，越是常用的介词，其含义越多。下面就简单介绍几组近义介词的用法及其搭配方法。 一. in, to, on和off在方位名词前的区别 1. in表示A地在B地范围之内。如： Taiwan is in the southeast of China. 2. to表示A地在B地范围之外，即二者之间有距离间隔。如： Japan lies to the east of China. 3. on表示A地与B地接壤、毗邻。如： North Korea is on the east of China. 4. off表示“离……一些距离或离……不远的海上”。如： They arrived at a house off the main road. New Zealand lies off the eastern coast of Australia. 二. at, in, on, by和through在表示时间上的区别 1. at指时间表示： （1）时间的一点、时刻等。如： They came home at sunrise （at noon, at midnight, at ten o’clock, at daybreak, at dawn）. （2）较短暂的一段时间。可指某个节日或被认为是一年中标志大事的日子。如： He went home at Christmas （at New Year, at the Spring Festival, at night）. 2. in指时间表示： （1）在某个较长的时间（如世纪、朝代、年、月、季节以及泛指的上午、下午或傍晚等）内。如： in 2004, in March, in spring, in the morning, in the evening, etc （2）在一段时间之后。一般情况下，用于将来时，谓语动词为瞬间动词，意为“在……以后”。如： He will arrive in two hours. 谓语动词为延续性动词时，in意为“在……以内”。如： These products will be produced in a month. 注意：after用于将来时间也指一段时间之后，但其后的时间是“一点”，而不是“一段”。如： He will arrive after two o’clock. 3. on指时间表示： （1）具体的时日和一个特定的时间，如某日、某节日、星期几等。如： On Christmas Day（On May 4th）, there will be a celebration. （2）在某个特定的早晨、下午或晚上。如： He arrived at 10 o’clock on the night of the 5th. （3）准时，按时。如： If the train should be on time, I should reach home before dark. 4. by指时间表示： （1）不迟于，在（某时）前。如：
介词用法口诀
介词用法口诀： 早、牛、晚要用in at黎明、午夜、点与分 年、月、年月、季节、周，阳光、灯、影、衣、帽in 将来时态in表以后，小处at大处in 有形with无形by 语言、单位、材展in 早、午、晚要用in in the morning 在早上 in the afternoon 在下午 in the evening 在晚上 in the day在白天 at黎明、午、夜、点与分 at dawn, at daybreak^ 黎明时候 at noon在中午at night在夜间 at midnight 在午夜 at six o'clock 在6 点钟 at 7:30 （seven thirty）在7 点半 at the weeken血周末 年、月、年月、季节、周都用in in 1986 在1986 年 in March在三月 in July, 1983 1983 年7 月i n spring在春季 in the third week 在第三周 阳光、灯、影（树荫）、衣、冒（雨）in Don't read in dim light.切勿在暗淡的灯光下看书。 They are sitting in the shade of a tree.他们坐在树阴下乘凉。 He went in the rain to meet me at the station. 他冒雨到车站去接我。 the woman in white （black, red, yellow）穿着白（黑、红、黄）色衣服的妇女in uniform 穿着制服 将来时态in表以后 They will come back in 10 days 他们将10 天以后回来。
介词的用法
（一）介词概述 介词是一种虚词，在句子中不单独作任何句子成分。它是一种表示名词和句中其他词之间关系的词。它常和名词、动词、形容词等搭配，构成固定短语，表示不同意思。介词还可以与名词构成介词短语，表示方位、方向、时间、地点、方式、原因等。这些介词短语在句中可充当定语、状语、补语等。2000—2005年的中考中主要考了形容词与介词的搭配：如：be famous for；表示时间的介词，如：at night；动词与介词的搭配，如：arrive in/have dinner with sb.；表示方式、手段的介词，如：by phone/in English. （二）基础知识梳理 1．名词与介词的搭配 a bit of有一点儿 a couple of两个、几个 a kind of一种、一类cover an area of占地面积 have pity on sb.怜悯某人huge amounts of大量的 make friends with与……交朋友make fun of拿……开玩笑 meet the needs of迎合……的需要one after another一个接一个；连续地 play a trick on捉弄the week after next下下周 2．动词与介词的搭配 agree with sb.同意某人的意见apologize to sb. for sth.为某事向某人道歉arrive at/in a place到达某地ask for请求、寻求 be covered with被……所覆盖be made of由……制成 be made up of由……组成belong to属于 break into破门而人、闯入 call on拜访 care for照顾、喜欢carry out执行 check in办理登机come across被理解；遇见 come from出生于、来自come on跟我来、走吧
地点和位置的介词
表示地点和位置的介词 1.at, in, on 1) at 表示较狭窄较小的地方（小村庄，小城镇），in 表示较大的地方（大城市，大的空间）。 I met her at the bus-stop. They arrived at the famous town in South Jiangsu. She is living in Nanjing. There are a great many islands in the Pacific. 2) 门牌号码前用at，road前面用on，street前用in或on。 in the street (BrE) on the street (AmE) at 103 Wall Street on the road 3) 把某个机构看成是机关或组织时用at，看成一个具体的地方时用in She is at Oxford. 她在牛津读书。She is in Oxford 她在牛津居住（工作或逗留） 4）at可以表示有意、有目的的行为。 She is at the table. 她在吃饭She is beside/ by the desk. 她坐在桌旁。 5) on 在---上面，表示上下两者紧贴在一起，in 表示在---里面。 She put the book on the desk. She put the book in the desk. She wore a smile on her face. (面部表情) She was wounded in the face. （伤有深度） 2．on 和underneath underneath 是on的反义词，表示某物紧贴在另一物的底下 There is a piece of paper underneath the dictionary. 3.under 和over under 和over 是一对反义词，表示正上方，正下方，没有接触的含义。 There are some chairs under the tree. The lamp hung over the table. under 和over 还可以表示上级、下级。 He is over us= We are under him. 4. above 和below 两者是一对反义词，表示高于、低于，既不表示接触，也不表示上下垂直。 The plane is flying above us. The sun has sunk below the horizon. 5. beneath 可以和underneath, under, below 互换。 6．at, by, beside at 表示有目的的接近、接触，by和beside 表示偶然的接近，不接触。 She will be waiting for you at the school gate at 7 tomorrow. The girl stood by/ beside her mother. To those who stand by me, I shall stand by my promise. 对那些支持我的人，我将恪守诺言。 7．near 和next to Near 表示在---附近，靠近；next to 表示紧挨着，紧靠着。 No birds or animals came near the lake. She went and sat next to him. 8. before, in front of, at the front of, ahead of before 用于某人前；in front of 用于建筑物前；in front of 和ahead of 用于空间可互换，用于时间只能用ahead of；in the front of 表示内部空间的前部；at the front of 表示外部空间
英语中表示时间、地点和位置的常用介词的应用 (自动保存的)
英语中表示时间、地点和位置的常用介词的应用 内容摘要：介词是一种用来表示词与词, 词与句之间的关系的词,在句中不能单独作句子成分。介词后面一般有名词代词或相当于名词的其他词类、短语或从句作它的宾语。介词在英语中有非常重要的地位,但其种类繁多,用法复杂,一个介词还往往有多种意义,本文将从时间、地点和用名词尤其是由动词转化过来的名词的机会较多,而名词与名词之间常常需要介词连接,故英语中使用介词的现象比比皆是。 关键词:表示时间地点和位置附加状语名词动词 内容： 一、英语中表示时间的常用介词 after表示...之后，指某事发生在所指时间后的任意时间，是before的反义词，表示在...之后（1）用在附加状语里，常跟有-ing小句，，如： Li Ming was released from prison after serving three years.李明在服刑三年后出狱。（2）用在名词之后，如：I hate the time after sunset before you come home.我不喜欢日落之后、你回家之前的这段时间。（3）用在动词be之后，如：He turned and went after his brothers.他转身去追赶他的兄弟们。after通常指次序的先后。 in（1）表示在某世纪、年、季度、月、周”以及泛指的上、下午、晚上。如：in the 20th century在20世纪；in 1999在1999年；in winter在冬季；in September在九月；in the morning/afternoon/evening 在上午/下午/晚上。（2）in 过...后(未来时间)，不晚于，如：The train will arrive in three hours.火车三小时到。（3）表示某段时间之后，如：to return in a few minutes/hours/days/months几分钟/几小时/几天/几个月后回来。（4）表示做...时，...发生时，当...时，如：In attempting to save the child from drowning, she nearly lost her own life.她在抢救落水儿童时，自己差点丧命。 注：after与in都可表示“在……之后”，但after后可跟时间段，也可跟时间点；而in后必须跟时间段。after既可用于将来时，也可用于过去时，而in只能用于将来时。I’ll arrive in an hour. 我一小时后到。 during表示在……期间(1)某事在某段时间里连续发生或发生过几次，用在附加状语里，如：During all the years of work, he had been realistic with himself.这些年来在所有的工作中，他总是对自己实事求是。（2）表示某物在某段时间里从开始到结束，都一直得到发展，用在附加状语里，如:I hope this will become clear to you during the course of the lectures.我希望在讲座过程中，你会渐渐明白这一点。（3）指某事在某一期间内的某一刻发生，用在附加状语里，如：The boy disappeared from the hotel during the night.这名男孩在夜间从旅馆失踪。 from表示时间的起点，从……起，多用于“from…to/till…”中。如：You can come anytime from Monday to Friday. 周一至周五你什么时间来都行。from 仅说明什么时候开始，不说明某动作或情况持续多久。 for 表示达...之久（1）过了多少时间，后接“一段时间”，表示某事持续多久，多与完成时连用，如：She has been ill for several days. 她已经病了几天了。（2）表示某事已经发生过一次，用在附加状语里，如：Before using a pan for the first time, wash it with a sponge.在首次使用平底锅前，用
英语介词的用法总结
介词的用法 1.表示地点位置的介词 1)at ,in, on, to，for at (1)表示在小地方; (2)表示“在……附近，旁边” in (1)表示在大地方; (2)表示“在…范围之内”。 on 表示毗邻，接壤，“在……上面”。 to 表示在……范围外，不强调是否接壤；或“到……” 2)above, over, on 在……上 above 指在……上方,不强调是否垂直，与below相对； over指垂直的上方,与under相对,但over与物体有一定的空间，不直接接触。 on表示某物体上面并与之接触。 The bird is flying above my head. There is a bridge over the river. He put his watch on the desk. 3)below, under 在……下面 under表示在…正下方 below表示在……下，不一定在正下方 There is a cat under the table. Please write your name below the line. 4)in front [frant]of, in the front of在……前面 in front of…意思是“在……前面”，指甲物在乙物之前，两者互不包括；其反义词是behind（在……的后面）。There are some flowers in front of the house.(房子前面有些花卉。) in the front of 意思是“在…..的前部”，即甲物在乙物的内部.反义词是at the back of…（在……范围内的后部）。 There is a blackboard in the front of our classroom. 我们的教室前边有一块黑板。 Our teacher stands in the front of the classroom. 我们的老师站在教室前.(老师在教室里) 5）beside，behind beside 表示在……旁边 behind 表示在……后面 2.表示时间的介词 1)in , on，at 在……时 in表示较长时间，如世纪、朝代、时代、年、季节、月及一般（非特指）的早、中、晚等。 如in the 20th century, in the 1950s, in 1989, in summer, in January, in the morning, in one’s life , in one’s thirties等。 on表示具体某一天及其早、中、晚。 如on May 1st, on Monday, on New Year’s Day, on a cold night in January, on a fine morning, on Sunday afternoon等。 at表示某一时刻或较短暂的时间，或泛指圣诞节，复活节等。 如at 3:20, at this time of year, at the beginning of, at the end of …, at the age of …, at Christmas，at night, at noon, at this moment等。 注意：在last, next, this, that, some, every 等词之前一律不用介词。如：We meet every day. 2)in, after 在……之后 “in +段时间”表示将来的一段时间以后； “after+段时间”表示过去的一段时间以后； “after+将来的时间点”表示将来的某一时刻以后。 3)from, since 自从…… from仅说明什么时候开始，不说明某动作或情况持续多久；
英语地点介词的正确使用方法
英语地点介词的正确使用方法 地点介词主要有at ，in，on，to，above，over，below，under，beside，behind ，between。它们的用法具体如下： 1、at （1）at通常指小地方：In the afternoon，he finally arrived at home。到下午他终于到家了。 （2）at通常所指范围不太明显，表示“在……附近，旁边”：The ball is at the corner。球搁在角落里。 2、in （1）in通常指大地方：When I was young，I lived in Beijing。我小时候住在北京。 （2）在内部：There is a ball in in the box。盒子里有只球。 （3）表示“在…范围之内”（是从属关系）： Guangdong lies in the south of China。深圳在中国的南部。 3、on
（1）on主要指“在……之上”，强调和表面接触： There is a book on the table。桌上有一本书。 （2）表示毗邻，接壤（是相邻关系）： Canada lies on the north of America 加拿大在美国的北边（与美国接壤）。 4、to 主要表示“在……范围外”，强调不接壤，不相邻。 Japan is to the east of China。日本在中国的东面。 注意： （1）at 强调“点”，on 强调“面”，in 强调“在里面”，to 表示“范围外”。 （2）on the tree：表示树上本身所长着的叶子、花、果实等 in the tree：表示某物或某人在树上 on the wall：表示在墙的表面，如图画、黑板等 in the wall：表示在墙的内部中，如门窗、钉子、洞、孔 5、above
表示地点位置的介词
表示地点位置的介词 w qsa 1)at ,in, on, to，for at (1)表示在小地方; (2)表示“在……附近，旁边”in (1)表示在大地方; (2)表示“在…范围之内”。on 表示毗邻，接壤，“在……上面”。to 表示在……范围外，不强调是否接壤；或“到……”2)above, over, on 在……上above 指在……上方,不强调是否垂直，与below相对；over指垂直的上方,与under 相对,但over与物体有一定的空间，不直接接触。on表示某物体上面并与之接触。The bird is flying above my head. There is a bridge over the river. He put his watch on the desk. 3)below, under 在……下面under表示在…正下方below表示在……下，不一定在正下方There is a cat under the table. Please write your name below the line. 4)in front [frant]of, in the front of在……前面in front of…意思是“在……前面”，指甲物在乙物之前，两者互不包括；其反义词是behind（在……的后面）。There are some flowers in front of the house.(房子前面有些花卉。) in the front of 意思是“在…..的前部”，即甲物在乙物的内部.反义词是at the back of…（在……范围内的后部）。There is a blackboard in the front of our classroom. 我们的教室前边有一块黑板。Our teacher stands in the front of the classroom. 我们的老师站在教室前.(老师在教室里) 5）beside，behind beside 表示在……旁边behind 表示在……后面 2.表示时间的介词 1)in , on，at 在……时in表示较长时间，如世纪、朝代、时代、年、季节、月及一般（非特指）的早、中、晚等。如in the 20th century, in the 1950s, in 1989, in summer, in January, in the morning, in one’s life , in one’s thirties等。on表示具体某一天及其早、中、晚。如on May 1st, on Monday, on New Year’s Day, on a cold night in January, on a fine morning, on Sunday afternoon等。at表示某一时刻或较短暂的时间，或泛指圣诞节，复活节等。如at 3:20, at this time of year, at the beginning of, at the end of …, at the age of …, at Christmas，at night, at noon, at this moment等。注意：在last, next, this, that, some, every 等词之前一律不用介词。如：We meet every day. 2)in, after 在……之后“in +段时间”表示将来的一段时间以后；“after+段时间”表示过去的一段时间以后；“after+将来的时间点”表示将来的某一时刻以后。3)from, since 自从……from 仅说明什么时候开始，不说明某动作或情况持续多久；since表示某动作或情况持续至说话时刻，通常与完成时连用。since表示"自（某具体时间）以来"，常用作完成时态谓语的时间状语。since liberation（1980）自从解放（1980年）以来They have been close friends since childhood．他们从小就是好朋友。（1）since the war是指"自从战争结束以来"，若指"自从战争开始以来"，须说"since the beginning of the war"。（2）不要将since与after混淆。比较：He has worked here since 1965．（指一段时间，强调时间段）自从1965年以来，他一直在这儿工作。He began to work here after 1965．（指一点时间，强调时间点）从1965年以后，他开始在这儿工作。4)after, behind 在……之后after主要用于表示时间；behind主要用于表示位置。时间名词前介词用法口诀年前周前要用in 具体日子要用on 遇到几号也用on 上午下午得是in 要说某日上下午用on换in记清楚午夜黄昏用at 黎明用它也不错at用在时分前说“差”可要用上to 说"过''要用past 3.表示运动方向的介词： across, through 通过，穿过across表示横过,即从物体表面通过，与on有关，为二
介词的用法及习题
第七单元介词 我们经常在名词或名词短语、代词或动名词前用介词表示人物、事件等与其它句子成分的关系。介词后面的名词或相当于名词的词语叫介词宾语。介词可表示地点、时间、比较、反对、原因、手段、所属、条件、让步、关于、对于、根据等。 二、介词的意义 1．表示时间的介词 in表示“在某一时间段”，或“在……某一时候”，如用在月、季、年份、时代、世纪等时间名词的前面，或用来泛指一天的某一段时间。 In July/summer/2000/ancient times/the 1999’s In the morning/afternoon/evening In也可以指“在……之后”，表示从说话起的若干时间内，如： The bus will be here in ten minutes. On表示“在特定的某一天”，也可用于带有修饰语的一天的某个时间段之前。如： on Saturday, on Saturday morning, on the morning of August 1st at表示“在某一时间点”，或用来表示不确定的时间和短期的假日、时节等。如： at six o’clock, at Easter 介词over, through (out)两者均指“经过的全部时间”。 Stay over the Christmas. 介词for, since for表示动作或状态延续的全部时间长度，为“长达……”之意；since用于指从过去特定的某个时刻到说话时为止的一段时间；两者往往用于完成时。 I have been there for six years. We have not seen each other since 1993. During指“在……时期/时间内”，必须以表示一段时间的词或词组作宾语。 She was ill for a week, and during that week she ate little. 2．表示地点的介词 介词at指小地点或集会场合；on表示线或面上的位置；in表示在立体、区域或环境内，特别是那些教大，能够容纳相应事物的环境。 He works at Peking University. Your radio is on the desk. The boat is in the lake. 3．表示原因的介词 for常常表示褒贬、奖惩的原因或心理原因。 4．表示目的的介词 for表示拟定的接收人或目的；to表示实际的接收人或目的。 I bought the gift for my little sister. I gave the gift to my little sister. 5．表示“关于……”的介词 一般about用于比较随便的谈话或非正式的文体；on用于正式的讲话、著作或报告中；7．表示价格的介词 at和for都可表示价格，at仅表示价格，for还表示“交换”，如： Eggs are sold at 95 cents a dozen here. I bought it for five pounds.
时间地点介词的用法
具体日期前用“on” 注意: 一、含有this, that, these, those, every, each 等的时间状语前不用介词。如： We are going to play football this afternoon. 今天下午我们打算踢足球。 His father goes to work early every day. 他爸爸每天很早去上班。They are working on the farm at the moment. 这几天他们正在农场干活。 二、all day, all week, all year 等由“all +表示时间的名词”构成的时间状语前不用介词。如： We stay at home and watch TV all day.我们整天呆在家里看电视。 三、由“some, any, one等＋表示时间的名词”构成的时间状语前不用介词。如： We can go to the Great Wall some day. 有一天我们会去长城的。 四、时间状语是today, tomorrow, tomorrow morning, tomorrow afternoon, tomorrow evening, the day after tomorrow (后天)等，其前不用介词。如：
What day is it today?今天星期几？ Who's on duty tomorrow? 明天谁值日？ MORE: at 表示时间的某一点 （节日或年龄、瞬间或短暂的时间） Your memory is always poor at this time. （表示一天中的某个时刻不用冠词） I got up at six in the mopning. on 表示某日或和某日连用的某个时间段 You were late on Monday last week. in 用于表示除日以外的某一时间段 （表示年、月、季节、世纪时代） Sorry, I am late, the frist time in May. in和at都可表示地点，而in表示的地点比at所表示的地点大
英语表示地点位置的介词
表示地点位置的介词 1)at ,in, on, to at (1)表示在小地方; (2)表示“在……附近，旁边”in (1)表示在大地方; (2)表示“在…范围之内”。on 表示毗邻，接壤，“在……上面”。 to 表示在……范围外，不强调是否接壤；或“到……” 2)above, over, on 在……上 above 指在……上方,不强调是否垂直，与below相对； over指垂直的上方,与under相对,但over与物体有一定的空间，不直接接触。 on表示某物体上面并与之接触。 The bird is flying above my head. There is a bridge over the river. He put his watch on the desk. 3)below, under 在……下面 under表示在…正下方 below表示在……下，不一定在正下方 There is a cat under the table. Please write your name below the line. 4)in front of, in the front of在……前面 in front of…意思是“在……前面”，指甲物在乙物之前，两者互不包括；其反义词是behind （在……的后面）。 There are some flowers in front of the house.(房子前面有些花卉。) in the front of 意思是“在…..的前部”，即甲物在乙物的内部.反义词是at the back of…（在……范围内的后部）。There is a blackboard in the front of our classroom. 我们的教室前边有一块黑板。Our teacher stands in the front of the classroom. 我们的老师站在教室前.(老师在教室里) 5）beside，behind beside 表示在……旁边 behind 表示在……后面 2.表示时间的介词 1)in , on，at 在……时 in表示较长时间，如世纪、朝代、时代、年、季节、月及一般（非特指）的早、中、晚等。 如in the 20th century, in the 1950s, in 1989, in summer, in January, in the morning, in one’s life , in one’s thirties等。 on表示具体某一天及其早、中、晚。 如on May 1st, on Monday, on New Year’s Day, on a cold night in January, on a fine morning, on Sunday afternoon等。 at表示某一时刻或较短暂的时间，或泛指圣诞节，复活节等。如at 3:20, at this time of year, at the beginning of, at the end of …, at the age
时间名词前介词用法口诀
英语中按动词后可否直接跟宾语，可以把动词分成两种：及物动词与不及物动词。及物动词：字典里词后标有vt. 的就是及物动词。及物动词后必须跟有动作的 对象（即宾语），并且可直接跟宾语。 如see 看见 (vt.) +宾语 I can see a boy. 不及物动词：字典里词后标有vi. 的就是不及物动词。不及物动词后不能直接 跟有动作的对象（即宾语）。若要跟宾语，必须先在其后添加上某个介词，如 to,of ,at后方可跟上宾语。具体每个动词后究竟加什么介词就得联系动词短语了，如listen to,look at…. 如：look 看 (vi.) x宾语(即不能直接加宾语). Look! She is singing. Look carefully! (注意：carefully 是副词，不是名词，故不作宾语) look at 看…….+宾语 Look at me carefully! (me是代词，作宾语) 介词是一种用来表示词与词, 词与句之间的关系的虚词。在句中不能单独作句子成分。介词后面一般有名词代词或相当于名词的其他词类，短语或从句作它的宾语。介词和它的宾语构成介词词组，在句中作状语，表语，补语或介词宾语。并且在定语从句“介词+whom/which”的结构中，不能用that 代替who/which。She is a good student from who we should learn. 介词分为时间介词.地点位置方向介词.方式介词.原因介词和其他介词，是英 语中最活跃的词类之一，特别是一些常用介词的搭配力特别强，可以用来表示各种不同的意思。英语里大部分习语都是由介词和其他词构成的。介词在句中一般不重读\ 表示地点位置的介词 1)at ,in, on, to，for at (1)表示在小地方; (2)表示“在……附近，旁边” in (1)表示在大地方; (2)表示“在…范围之内”。 on 表示毗邻，接壤，“在……上面”。 to 表示在……范围外，不强调是否接壤；或“到……” 2)above, over, on 在……上 above 指在……上方,不强调是否垂直，与 below相对； over指垂直的上方,与under相对,但over与物体有一定的空间，不直接接触。 on表示某物体上面并与之接触。 The bird is flying above my head. There is a bridge over the river. He put his watch on the desk. 3)below, under 在……下面 under表示在…正下方 below表示在……下，不一定在正下方 There is a cat under the table. Please write your name below the line.