文档介绍：关于情态动词语法讲解
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情态动词（modal verbs)：
表示说话人的语气或情态，如请求、命令、愿望、能力以及可能性等。英语的情态动词有：
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can/ health. He can be ill at any time.
Mr Reed looks pale. He may be ill.
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“may/might +have done/have been doing”,“can/could +have done”表示过去的可能。
He may/might have missed the train.
She may/might have been expressing her true feelings at that moment.
She can’t/couldn’t have missed the train.
“might/could +have done ”可以表示本来可能发生但没有发生的，或者本来可能完成却没有完成的动作，
You might have finished the work last week.
I could have passed the examination, but I failed.
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2表示“许可”和“不许”
a)请求对方“许可”可用can, could, may, might.
may/might较正式，could/might较委婉
表示给予“许可”通常用can/may,而不用could/might
Could I use your phone?
Yes, of course you can.
Might I trouble you for a light?
You may indeed.
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b)表示“不许”可用may not或者cannot
Can I go out for a moment?
No, you can’t.
May I use your car for a few days?
No, you may not.
may not也可表示根据一般规定的“不许可”，而不是说话人不许可
Borrowers may not take out of the library more than two books at a time.
may not表示“不许可”的过去式不是might not
He may not go.(=I don’t permit him to go.)
He might not go.(=Possibly he will not go.)
过去的不许可：He was not allowed to go.
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3表示“义务”和“必然”
a)表示“义务”，可用should, ought to, must.
表示要求，命令时，语气由ought to、 should、must渐强
Should/ought to表示“应该”，带有敦促，劝说之意，可交替使用，前者语气更强。
You should/ought to drink less.
Should/ought to +have done:本应该……
You should/ought to have asked my permission first.
Must表示敦促或命令，必须。
I really must stop smoking.
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表示将来的“必须”，常用have to的一定形式（will/shall have to) 比较：
We must do it again.(表示现在)
We’ll have to do it again.(表示将来)
表示过去的“必须”，常用had to
I had to leave at six yesterday.
must有两种否定形式:第一个是must not,表示“不许”、“禁止”,相应的半助动词是be to的否定形式be not to
You mustn’t talk like that.
You are not to talk like that.(不许你如此讲话。)
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第二个否定形式是needn’t,表示不必。
与needn’t相当的动词形式是don’t need to/don’t have to/haven’t got to
A: Must you leave soon?
B: No, I needn’t/don’t need