Modal Verbs
情态动词有can (could), may (might), must (must), have to, shall (should), will (would), need (need), dare (dared), ought to 等。情态动词无人称和数的变化。它不能单独使用，必须跟其后的动词原形构成谓语。情态动词的具体用法如下：
一、can, could
1、表示能力。
a. Can you speak English?
b. Can you finish this work tonight?
c. Man cannot live without air. Note:
(1) can表示能力时，可用be able to 代替。
a. I’ll not be able to come this afternoon.
(2) 当我们要表示“某件事情已经
成功”时，应用was/ were able to , 不能用could
a. He saw well and he was able to swim to the river when the flood happened.
b. He was able to go to the party yesterday evening and he enjoyed himself very much.
2、表示客观可能性
a. People who live near airports can have their hearing harmed.
b. The boy can sometimes be very naughty. （表示某人或某物一时的情况，“有时会。。。”）
c. The invention can be improve
d.
d. Even experts can make mistakes.
e. He can’t be in Beijing now because I saw him a moment ago.
f. He had a lot of work to do last night, so he couldn’t have gone to see a film.
Note:
在肯定句中用can表示可能性时，其含义是理论上的可能性，不表示
实际的可能性，也就是说，不涉及到是否真会发生。
3、表示惊异、怀疑、不相信的态
度（主要用与否定句、疑问句或感叹句中）
a. Can this be true?
b. How can you be so careless!
c. This cannot be done by him.
4、表示请求和允许。
a. --- Can I go now?
--- Yes, you can.
b. You can smoke if you want to.
c. You can’t pick flowers in this park. Note:
could也可以表示请求，语气较委婉，主要用于疑问句，不用于肯定句，回答应用can
--- Could I come to see you tomorrow?
--- Yes, you can. / No, I’m afraid not.
5、can 的一些其他惯用法
(1). cannot but, cannot help but, cannot choose but +动词原形，意思为“不得不，只得”。
a. I cannot help but tell her the truth.
2. cannot help doing sth
a. I cannot help laughing.
3. cannot …too / enough, can never too
a. You cannot be too careful when you cross the street.
b. I cannot thank you too much for your kindness; I owe my progress to you.
二、may, might
1、表示许可。
a. You may go now.
b. We may keep the books for two weeks.
c. --- May I smoke here?
--- Yes, you may. / No, you may not. / No, you mustn’t.
2、表示可能性
a. He may be waiting for you at the station.
b. They may have got lost.
Note:
表示可能性时，may 不用于疑问句。在疑问句中表示“可能”时，
通常用“be likely to ”或“Do you think”这一结构。
3、用于让步状语从句中
a. However hard you may study, you cannot master English in a month.
b. Don’t give up whatever difficulties you may meet with.
4、用于祈使句，表示祝愿。
a. May you succeed!
b. May we never forget each other!
5、might常用于表示委婉的请求或
轻微的责备。
a. You might post this letter for me if you are going near a post box.
b. You might have let me know before!
６、用于某些习惯用法中
(1) may /might as well = had better
a. It’s getting darker and darker. You may/might as well go home.
(2) may well + 动词原形，意思为
