英语语法情态动词精讲

　   1. 情态动词的推测表达
　　2. 情态动词表达虚拟语气
　　3. 某些情态动词的特殊用法
　　知识重点与难点总结
　　知识重点：
　　情态动词在表达推测意义的句子中的运用：
　　（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。
　　（二）对现在的事实进行推测：
　　主要结构：…must / may / might +动词原形
　　                             be+名词/形容词/介词短语
　　                             be + doing
　　例句：
　　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
　　例句：
　　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……
　　could have done：本来可以……
　　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……
　　例句：
　　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.
　　知识难点：
　　某些情态动词的特殊用法：
　　need 和dare 的两种形式的用法
　　need 和dare可以用做实义动词，后面接不定式（to do）结构，在疑问句和否定句中，加助动词do/does/did/或don’t/doesn’t/didn’t。作为情态动词使用时，主要用于疑问句和否定句中。情态动词needn’t（没有必要，不必）相当于don’t have to
　　例句：
　　1. It is cold, you need to wear some warm clothes.
　　2. Need I stay here with you for a while ?   Thank you, you needn’t.
　　3. How dare you speak to parents like that ?
　　注意：
　　句型I dare say+从句。 意思是：我肯定…… = I’m sure或There is no doubt that+从句。
　　例句：
　　I dare say that my uncle will get the money if I die.
　　will和would
　　表示“意志”或“愿意”，would 则指过去愿意做……
　　例句：
　　1. He said that he would help us.
　　2. You may telephone if you will accept this job.
　　would可以表达“过去习惯做……”类似于“used to do”
　　例句：
　　1. When we were children, we would go swimming every summer in that river.
　　表示请求，固定的句型：Will/Would you please do…? / Would you like to do…?
　　例句：Will/Would you please give him a message when you see him ?
　　shall
　　1. 用于第一，三人称，表示“请求”；“建议” 或“推荐”
　　例句：
　　1. Shall we start the meeting now?
　　2. Shall I watch TV now ?
　　3. Shall my son carry the case for you ?
　　2. 用于第二人称，表示“命令，“要求”，“许诺”
　　例句：
　　1. You shall take whatever you like.
　　2. You shall not go to the party with me if you make so much noise again.
　　在表示推测的否定句或疑问句中，常用can /can’t /could / couldn’t表示，意思是：“可能…吗?”；“…不可能…”。而不能使用mustn’t或must等词。
　　【典型例题】
　　1. —Do you think he will do me a favor ?
　　 —As far as I know, he is the last one to help others. He _____ be prepared to give you a hand, though.
　　A. might  B. must   C. can D. should
　　分析：本题考查情态动词推测性用法。 根据he is the last one to help others.（他是最不可能帮助别人的人），既然如此，那么“帮助你”的可能性也就最小了。 答案为A
　　2. —Look, someone is coming. Guess who it ____ be ?
　　 —I think it ___ be Tom.
　　 —I don’t think it ___ be ____ .
　　A. can ; must ; can ; he    B. may ; can ; must ; him
　　C. must ; can ; must ; his    D. might ; must ; can ; himself
　　分析：根据备选答案。 can表示推测时用于疑问句或否定句中。本题首句就应该是Guess who can it be? 第二句应该是I think it must be Tom.（说话人十分肯定）。第三句表达了说话人对前一个人的否定。即：I don’t think it can be him/he.（不可能是他）。答案为A
　　3. —Do you know Ms. Wang likes walking after supper ?
　　 —Sure. She ____ around the campus now.
　　A. must be walking     B. must walk
　　C. may walk      D. may be walking
　　分析：关键词Sure告诉我们说话人的口气十分肯定。根据句子语境：她此刻肯定正在校园散步呢。must be doing表示肯定正在做。答案为A
　　4. —I stayed at a hotel in New York.
　　 —Oh, did you ? You _____ with Barbara.
　　A. could have stayed B. could stay     C. would say  D. must have stayed
　　分析：首句说：我在纽约住在一家宾馆里。在此a hotel是泛指。所以D选项不合题意。第二句在说：你本来可以和Barbara.住在一起的。Could have done表示：本来可以。答案为A
　　5. Why didn’t you tell me there was no meeting today ? I _____ all the way here through the heavy snow.
　　A. needn’t have driven    B. can’t have driven
　　C. mustn’t have driven    D. shouldn’t have driven
　　分析：根据句意：你为什么不告诉我今天没有会？我本来没必要冒着大雪开车跑这么多路。Needn’t have done表示本来没必要。答案为A
　　6. I was really anxious about you. You _____ home without a word.
　　A. mustn’t leave      B. shouldn’t have left
　　C. couldn’t have left     D. needn’t leave
　　分析：根据句意：我真的很担心你，你真不应该一句话都不说就离开家。“离开家”已经发生了。Should have done正好表达了本句的意思。答案为B
　　7. A：Are you coming to Jeff’s party ?
　　    B：I’m not sure. I _____ go to the concert instead.
　　A. must  B. would  C. should  D. might
　　分析：根据B的回答：我不一定去，我有可能去听音乐会。Might表示推测：可能。答案为D
　　8. A：I promise that she ____ get a nice present on her birthday.
　　    B：Will it be a big surprise to her ?
　　A. should  B. must  C. would  D. shall
　　分析：A说：“我答应她在生日聚会上她会得到一份生日礼物。shall表示许诺。答案为D