情态动词 语法讲解
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1 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: 2 (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 3 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 improved.
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 4 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: 5 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. 6 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. 7 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. 8 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 9 “理所当然，有足够的理由”。
a. He has grown up tall. You may
well not recognize him.
三、must, have to
１、表示义务和强烈的劝告。意思为“必须”、“一定要”。
a. You must finish your task ahead of
time.
b. You must talk to your daughter
about her future.
2、表示有把握的推测，意思为“一定是”，“准是”。
a. I failed in the exam. You must 10 think I am stupid.
He must be a college student,
isn’t he?
b. You look happy. You must be
having a good time.
c. He looks tired. He must have
stayed up late last night, didn’t he?
d. You speak English so fluently. You
must have learned it for many years,
haven’t you?
3、表示肯定性或难以避免，意思为“必然会、”“肯定会”。
a. All men must die.
b. Don’t bet o horse races; you must
lose in the long run.
4、作“偏要”、“硬要”解。 11 （1）常以第二人称为主语，意思指不耐烦过令人不愉快的事情。
a. If you must smoke, at least you
should do it outside.
b. If you must know, I’m going to
help him look for an apartment.
c. Why must you buy that car?
（2）用于其它人称，表示主语“固执”或“不巧”的意思。
a. The car must break down just as
we were starting our holiday.
b. Jane was never a pleasant young
girl. After you gave her your
advice, she must go and do the
opposite. 12 5、must的否定有如下三种形式，用于三种不同的场合。
(1) 当must表示“推测”时，其否定含义为“不可能”。将 must
be 改为 cannot be；将 must
have done 改为 cannot have
done。
a. It must be eleven o’clock now.
It cannot be eleven o’clock.
b. You must have met him before.
You cannot have met him before.
(2)当 must 表示“必须”时，其否定含义为“不必”。将 must do
