(一)用情态动词表达事实的推测。can，could，might，may，must可以用来表达对事实的推测。根据说话人对事实的把握性大小，must表示"肯定…"，may / might / can / could表示"可能……"， must只用于肯定句中，may / might的否定式may not / might not表示 "可能不……", 而can / could可以用于疑问句，表示"可能，可能……吗?"，其否定式can’t / couldn’t 表示"不可能"。用情态动词可以对现在或过去的事实进行推测。对不同的时间内容推测有不同的结构。
主要结构：…must / may / might +动词原形
1. You must be Jeanne. I’m Mathilde Loisel. We used to know each other very well.
2. They must be in bed already at this time of the night.
3. The teacher must be joking.
4. Freda isn’t in class. She must be sick.
5. There must be something wrong.
6. She might be very clever, but she hasn’t got much common sense.
7. He may be arriving this evening.
8. He may be traveling around the world.
9. The keys can’t be in the room. I have just searched it very carefully.
10. Can the news be true ?
(三)对过去的事实进行推测：结构：情态动词+have done / been+名词/形容词/介词短语
1. Mrs. Longmans must have been a pretty girl in her youth.
2. He couldn’t have seen Anna yesterday. She’s gone abroad.
3. I think I must have left my glasses in the library.
4. He might have overslept again.
5. Where can Tom have gone ?
情态动词表达虚拟语气：表达"本来…"，"不然早就…"。这是情态动词的一种虚拟语气用法。表示说话人所讲的与所发生的事实相反。表达了说话人的埋怨，后悔的语气。其结构是在一些情态动词后面加 have done 结构。根据要表达的意思，有如下结构：
should have done / ought to have done：本应该……
shouldn’t have done / oughtn’t to have done：本不该……
needn’t have done：本来没必要……
would like to have done：本来很想……
would rather not have done: 本来不愿意……
could / might / have done: 不然早就……
1. You shouldn’t have laughed at his mistakes.
2. You could have told us earlier.
3. I ought to have bought that dictionary last week.
4. You ought not to have spent so much time in reading novels.
5. We need not have been in a tearing hurry to catch the train.
6. They would like to have seen that film last film.
7. If he had given me his number, I could have telephoned him.
8. They might have been frozen to death but for the rescue in time.
