情态动词:

　　may/might表示允许和可能:

　　允许: 询问或说明一件事可不可以做.
　　May I trouble you with a question?
　　You may take this seat if you like.
　　He asked if he might glance through my album.
　　You might as well speak your mind. (比may…显得婉转一些)

　　可能: 表示一件事或许发生(或是某种情况可能会存在).

　　You may walk ten miles without seeing a house.
　　She was afraid they might not like the idea.
　　A bad thing might be turned to good account.
　　might 表示请求:
　　Might I ask for a photograph of your baby? (比May I …更客气一些)

　　can’t, couldn’t表示否定的推测:

　　She can’t be serious.
　　A more suitable book can’t be found. (It is not possible to find a more suitable book.)
　　He couldn’t (can’t) be over fifty.
　　should, ought to: 表示应该做的事, ought to比should口气稍重一些.
　　You should (ought to) do as he says.
　　You shouldn’t (oughtn’t to) talk like that.

　　但这两者间也有一些差别, 在表示责任, 义务等该做的事情时, 常用ought to, 在表示某件事宜于做时, 多用should, 在下面的句子中这两个词就不宜换用:

　　You are his father. You ought to take care of him.
　　We should not use too many big words in our everyday speech.
　　will, would
　　shall, should表示意愿

　　情态动词后接进行式, 完成式和完成进行式:
　　情态动词可以和动词的进行式构成谓语, 表示”应该正在…”, “想必正在…”这类意思:

　　Why should we be sitting here doing nothing?
　　This isn’t what I ought to be doing.
　　She might still be thinking about the question you raised.
　　They must (may) be waiting for us, let’s hurry up.
　　They can’t be using the room now.

　　情态动词有时和动词的完成形式构成谓语, 表示”应当已经…”, “想必已经…”这类意思:

　　I should have thought of that.
　　They shouldn’t have left so soon.
　　She must have arrived by now.来源：上考试大网校，考试轻松过关
　　You needn’t have told them that.
　　Where can (could) he have gone?
　　He can’t have finished the work so soon.
　　He may not have achieved all his aims. But his effort is praise-worthy.
　　We ought to have give you more help.

　　情态动词间或也可以和一个动词的完成进行式构成谓语,表示”应当一直在…”, “想必一直在…”这类意思:

　　They are sweating all over. They must have been working in the fields.
　　They may have been discussing the problem this morning.
　　You should have been waiting for us. Why haven’t you?
　　She couldn’t have been swimming all day.