英语六级语法精要情态动词

　　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.

　　可能: 表示一件事或许发生.

　　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?

　　cant, couldnt表示否定的推测:

　　She cant be serious.

　　A more suitable book cant be found.

　　He couldnt 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 do as he says.

　　You shouldnt 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 isnt what I ought to be doing.

　　She might still be thinking about the question you raised.

　　They must be waiting for us, lets hurry up.

　　They cant 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 shouldnt have left so soon.

　　She must have arrived by now.

　　You neednt have told them that.

　　Where can he have gone?

　　He cant 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 havent you?

　　She couldnt have been swimming all day.