情态动词详解及举例
情态动词只有情态意义，即它所表示的是说话人对动作的观点，如：需要、可能、意愿或怀疑。
情态动词有以下特征：在形式上，情态动词没有实义动词的各种变化，只有could, would, had to, might等几个过去式。其他的情态动词如must, ought to等的过去式皆与现在式同形。
在意义上，大多数情态动词有多个意义，如can表示“能够” “可能”“允许”may表示“可能”“允许”“目的”“让步”等。
在用法上，情态动词与助动词一样，后面必须加动词原形，构成谓语动词。
Can/could
He can speak English. Can you come to the meeting?
Could多指的是过去式，多用于过去。
He could speak English when he was a child.
Could I help you? (表示委婉语气)
Can还能够表示能力
Can you use chopsticks?
I can lift the stone.
May / might 表示可能，暗含不确定，等于possibly, perhaps或者maybe
It may be true. He told me it might be true.
Might这个词表示也许，语气比may还要委婉，缓和，含义也更加不确定
It might be true. May I come to see you?
He may be busy getting ready for his trip.
而May也可以用在肯定句或否定句里。
He may be at home. He may not be at home.
也可以表将来 He may come soon.
表祝福。 May the happiness be with you all the time.
Must的形式和用法
Must 在肯定结构中有表示义务，坚定的建议、可能性很大的推测等。在否定句中表示不许做某件事。
You must talk to your daughter about her future.
Must he do it himself?
You must come and see us when you’re in London.
He must not leave his room.
Have to 表示客观的义务或者需要。可用于各种句式。但只是用于一般式。
He had to do it. Did he have to do it?
在否定句当中，have to表示不需要，must 表示不许。
You don’t have to go there. You mustn’t go there.
而固定词组had better 表示可取或者应该or最好。
You’d better get some sleep.
What had we better do?
Ought to 表示义务，表示可能性很大的推测，但语气比must弱。
Ask John, he ought to know.
You oughtn’t smoke so much.
Shall和should
Shall表示义务，用于第二第三人称。
You shall do as you see me to.
He shall be punished if he disobeys.
Shall一般也多用于法律文字。
There shall be no adultery.
Should表示义务上的，表示有把握或者证据的推测，
You should do what your parents tell you.
Should I open the window? (表示用第一人称询问时表示征询意见。
Will和would
都表示意愿，都用于第一人称，will指的是将来。Would指的是过去的将来。
I won’t argue with you.
I said I would do anything for him.
如果主语是东西，will和would则表示不起作用。
My drawer won’t shut.
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