①不定式作主语一般表示特定的、具体的动作，也往往用形式主语It is …to do sth的形式表达；而
动名词作主语则表示抽象的、泛指的动作，只有It is +否定意义的形容词或名词(no use, no good)的句型中用形式主语。
It is a pleasure to help others. 帮助别人是一件乐事。
It is no good doing nothing all day long. 整天无所事事不好。
Y our saying that is useless. 你那么说没有任何用处。
To eat too much sugar is bad for your health. 多吃糖对身体有害。
（正）Y our saying that is useful.
To see is to believe. 眼见为实。
Teaching is learning. 教学相长。
To waste time is to waste life. 浪费时间就是在浪费生命。
Her job is washing clothes. 她的工作是洗衣服。
上句还可表示为Washing clothes is her job.
His wish is to become an artist. 他的愿望是成为一名艺术家。（表将来）
To do first aid correctly, you have to study with a teacher.
In order not to waste electricity, we turned the light off.
Check your composition carefully so as to avoid mistakes. (so as to只能置于主句之后)
The two friends were pleased to see each other again.
Nice to meet you again.
They lifted a rock only to drop it on their own feet. 搬起石头，砸自己的脚。
He worked hard only to fail. 他很努力，但结果还是失败了。
I woke early to find it was snowing. 我早早醒来，看到在下雪。
Y ou are too young to go to school. 你年纪太小了，不能上学。
The question is too difficult to answer.这个问题太难了，无法回答。
（2）当too前出现only, but, all等副词时，too…to…句型不再表示否定的意义，only(but, all) to o…相当于really，too没有否定的含义，相当于very.
I am only too glad to receive your nice gift. 我非常高兴地受到你的精美礼物。
We are but too eager to hear from you. 我们及其渴望受到你的来信。
(3)当句型中的形容词是表示“难”“易”或表示某种心情的形容词，如ready, easy, glad, pleased等时，too作“十分，极其”解，表示肯定的意义。
She is too ready to help others. 她很乐于助人。
His mother is too easy to get angry. 他母亲太容易发脾气了。
（4）never(not)+to o…to…句型为“不会太…所以能…”之意。否定词never(not)与含否定意义的to o…to…句型一起构成双重否定，即为肯定含义。
One is never too old to learn. 活到老，学到老。
It is never too late to mend. 亡羊补牢，未为晚也。
She did not get up too late to catch the early bus. 她起得早，赶上了早班车。
(5)t oo…not t o…句型表示肯定含义，意义为“太…不会不…”。
He is too kind not to help you. 他这么好，不会不帮助你的。
5.关于to have done(不定式的完成式)
（1）不定时的完成式与表示愿望的动词如expect, intend, wish, plan, want, mean, suppose的过去时以及would like/love等连用，表示过去未曾实现的愿望。
I meant to have given you the dictionary yesterday, but I forgot to bring it here.
He intended to have written the manager a letter this morning, but he was too busy.
I’m sorry to have taken up so much of your time. 很抱歉我占了你如此多的时间。
She is said to have learned several languages. 据说她已经学了几种语言。
We expected to have finished the task by the end of next month.
We’re leaving at 8, and hoped to have reached there by sunset.
(1)在使役动词let, have, make 和感官动词feel, see, watch, notice, observe, listen to, hear后作宾语补足语，不定式的符号to要省略。
Mr Wang has his son write a diary every day. 王先生让儿子每天写一篇日记。
I knew him very much because I had seen him grow up from childhood.
There was nothing I could do but wait.
She would do anything except clean the floor.
（3）两个或两个以上功能相同的不定式并列使用时，第一个to要加上，之后的不定式符号to要省略. The teacher asked us to bring some food, wear more clothes and get there on time.
To make him happy and encourage him to work harder, I gave him a walkman as a gift.
（4）为了避免重复，在hope, wish, want, like, love, decide, plan, need, mean, forget, refuse, tell, know,
haveto, be going to, used to, ought to等动词后面再次出现相同的不定式作宾语时，常出现单独使用的to，而把曾出现过的动词省略掉。
①Mrs King lied to us because she had to. 金女士跟我们撒了谎，因为她不得不这样做。
②---Would you like to go skating with us? 和我们去滑冰好吗？
---I’d like /love to, but I have a lot of homework to do.我想去，但我有许多作业要做。
③---I heard that you would go to Dalian for your holiday. 听说你要到大连去度假？
---I planned to, but I have some important business to deal with. 原计划去的，但我有重要的事要处理。（5）不定式在afraid, anxious, able, eager, glad, lucky, sorry, willing等形容词后面出现且有上下文时，常出现单独使用的to，而把曾出现过的动词省略掉。
----I’ll be away on a business trip. Would you mind looking after my cat?
----Not at all. I’d be glad to.行，非常乐意。
（6）在ask, tell, expect, force, invite, beg后面做宾补时，常出现单独使用的to，而把曾出现的动词省略掉，以免重复。
John didn’t pass his driving test, but I expected him to.
Jane came to my birthday party last night, but I didn’t invite her to.
(7)但是，如果在省略的不定式结构中含有be, have, have been时，要保留这些词。
①----Hasn’t he finished writing the report?
----No, but he ought to have.
②----Aren’t you the headmaster?
----No, and I don’t want to be.
③China is no longer what it used to be.
Do you have a knife to cut the watermelon?
She has a sister to look after.
（3）在形容词+不定式结构作表语，宾补和定语时，其中形容词多为表示性质的词，如：easy, hard, difficult, interesting etc.
These apples are good to eat.
I find the lecture difficult to understand.
He is a man easy to get on with.
The passage is too hard to translate.
I don’t know what to do next.
She will tell you which bus to take. Do you know who to go with?
（6）在不定式作定语，与被修饰词之间有动宾关系，而与句中的另一名词或代词之间有主谓关系时I have a meeting to attend today.
Do you have anything to say?
I’m going to Beijing next week. Do you have anything to be taken to your parents?
（7）在下列句型中，to let, to blame, to seek等用主动形式表被动含义。
The house is to let.
Who is to blame for it?
The reason is not far to seek. 理由不难找到。
There is no time to lose. There is no time to be lost.
There are many problems for us to deal with.
There are many reasons for the books to be published.
1. The pilot asked all the passengers on board to remain _______ as the plane was making a landing.
D. to be seating
2. Don’t leave the water ______ while you brush your teeth.
C. being run
D. to run
①leave sb/sth doing sth
②leave sth undone
③leave sb to do sth
④leave sth to be done
3. The old man, ____ abroad for twenty years, is on the way back to his motherland.
A. to work
C. to have worked
D. having worked
4. I send you 100 dollars today, the rest _____ in a year.
C. to follow
D. being followed
解题思路：此题考查独立主格用法。逻辑主语the rest与follow之间的关系为逻辑上的主动关系。而A项follows为谓语动词，题干句逗号后没有连词不能跟句子。句中的in a year暗示将来，所以用不定式表将来，与the rest构成独立主格结构。
5. oil prices have risen by 32 percent since the start of the year, ______ a record US＄57.65 a barrel on
A. have reached
C. to reach
D. to be reaching