在表达“能力”时，可用 be able to 来取代 can。要注意的是：可在 be able to 前加肋动词或情态动词以表达更多的时态或情态，can 或 could 前则不可。
No one is able to do it. ( = No one can do it.)
We shall be able to finish the work next week.
I haven't been able to find any jobs.
You may go home now.
He said th at I might drink some beer.
May I watch TV this evening?
--- May I use you pen?
--- Yes, you may.
He may/might come tomorrow.
1. 表达“义务”、“责任” 。
3. 其否定式 mustn't 表示“（说话人）禁止……，不准……”；用来代替 may not
Everybody must obey the rules.
I must go now.
This work must be finished as soon as possib le.
The Chinese language must have the largest number of speakers.
You mustn't lend the book to others.
在表示“客观需要”时，用 have to 而不用 must。另外可在 have to 前加肋动词或情态动词以构成更多的形式。must 前则不可。
You don't have to worry about that.
He had to spend his childhood in hard work, hel ping his father on their small farm.
The students of today will have to how to use the computers.
We should be strict in all our work.
Where shall I wait for you?
Shall we start the meeting now?
Tell him that he sha ll have the book tomorrow.
You shall do as I say.
Young people should learn how to use computers.
在表示“必要性”时，可用 ought ot 来代替 shall，语气更正式一些。
Fi sh will die out of water.
Everyday she would get up at 6:00 and light the fire.
I will tell you all about it.
We'll hel p him if he asks us to.
I won't go home now.
I promised that I w ould do my best. Would you like another cup of tea? Will you go with me?
Will you please ... ?