情态动词
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第十二章情态动词本章难点:情态动词表示对具体事情的推测和客观可能性。
12.1 概述情态动词表示说话人对所述的动作或状态的看法,如请求、拒绝、必须、需要、可能、意愿、怀疑等情态意义,有一定的词义,其后要跟一个实义动词或系动词的原形构成谓语。
情态动词没有人称和数的变化。
有的有过去时态。
可以相当于一个助动词构成疑问或否定。
情态动词有:can(could), may(might), will(would),shall(should), dare, must, ought to, need, used to等13个。
下面将情态动词的具体用法。
12.2 can/could 的用法1. can 表客观可能性。
如:Even in summer it can be very cold there.Tigers can be dangerous.Y ours passage can be better.2. 表能力,与be able to同义,在许多的情况下可以交换使用。
Mary can (is able to) speak two foreign languages.I can (am able to) help you.注:①be able to 只能是人或动物作主语,以物作主语要用can。
如:He is able to/can swim.The book can be finished in an hour.The hall can hold four hundred people.②有助动词的时候只能用be able to。
He will be able to speak English.She won’t be able to get the job.③表过去的能力,在肯定句中,could泛指一般性的能力,be able to表过去做具体某件事的能力,相当于manager to do或succeeded in doing。
如:She could play the piano when she was only six. 她只有六岁的时候就会弹琴。
He was able to swim across the river when he was six. 他六岁的时候游过了这条河。
3. 表许可。
can 和could都可以用于现在时的疑问句征求对方许可,但肯定回答(给予许可)往往用can。
表许可也可以用may, might (具体用法请看may/might)。
在宾语从句里,为了时态一致,要用could表许可。
Y ou can’t go home now.Can/Could I have a few words with you? Y es, you can.Could/Can I sit here? Y es, you can.The police told him that he couldn’t leave his car on the road.4. 表请求,这时用could没有过去的意思,而是语气更加客气。
如:Can’t you be quite for a while?Could you do me a favor?Could you come and see me tomorrow?5. 表吃惊,怀疑,厌恶等。
如:What can he mean?How can you be so rude?Can it be true?6. 表推测。
对具体事情进行推测,can只能用于否定或疑问,肯定用could(也可以用may/might)。
如:According to the radio it could rain this evening.Who can that be at the door?It could be Michael.He can’t be more than thirty years old.can’t +have +done一般用来表对过去事情否定推测,译为“不可能”。
如:He can’t have said so.They can’t have been here yesterday.注:could+ have+ done表虚拟,指过去本来可以做而没做,表示一种遗憾或后悔等。
Y ou could have tried the back door.Y ou could have reported to him.Y ou could have finished the work last week.12.3 may/might的用法1. 表许可。
也可以用can/could表示。
但may/might用于正式场合,如上对下等;can/could 用于非正式场合,如朋友、同辈等。
may/can与might/could相比,后者更有礼貌,但表示过去许可只能用could或might。
如:May I come in?Y ou may smoke in the apartment.He asked if he might leave the office.I wondered if I might borrow some money.Might I ask your name?表不许可用may not,也可以用cannot。
may not表示按规定不许,cannot表示说话人不许。
如:Can I go out for a moment?No, you can’t.Can I use your car for a few days?No, you may not.2. 表推测。
may/might do(be doing)表示对现在或将来的推测。
否定推测一般用can’t/couldn’t,意思是“不可能”。
也有用may not/might not的,意思是“可能不”。
如:If you don’t work hard, you may fail again.He may not be serious.He told me that she might be the right person for the job.She might be watching TV in her room now.They can’t be having a meeting.May/might have done 表示对过去事情的推测,其反意问句根据情况可用助动词did或have。
如:He may have finished his work by now, hasn’t he?She might have learned more if she had made better use of her time, hasn’t she?They may have played tennis yesterday, didn’t they?12.4 must的用法1. 表必须做的事。
其疑问句的否定回答用needn’t或don’t have to。
如:We must do everything step by step.Must we hand in the paper this week?Y es, you must. (No, you needn’t. Or: No, you don’t have to.)注:must 与have to, 前者强调主观看法,后者着重客观需要。
如:I must get up at seven o’clock. (我必须7点钟起床。
强调主观认为)I have to get up at seven o’clock(我不得不7点钟起床。
强调客观要求)Children must obey their parents. (小孩必须服从父母。
主观认为)They have to obey their parents. (他们只有服父母。
客观要求)must只有现在时,表将来或过去时一般用have to。
如:I forgot the key so I had to wait till my wife came back.We will have to start early tomorrow.2. 表推测(只用于肯定),用must比用may/might的可能性更大。
如:This must be your bag.There must be a mistake.Y ou must be wrong.must have done 表对过去事情的推测,其反意疑问句不能用must来构成,根据情况,可用did或have来构成。
如:He must have done this project, hasn’t he?They must have played tennis yesterday, didn’t they?3. 表“偏偏”。
如:The car must break down just when we were about to start off.At a time when everybody was in bed, he must turn on the radio.12.5 ought to 与should的用法1. 表应该做的事。
两者都可,当ought to比should语气更强,或者说ought to更客观,should更主观。
如:We ought to (should) help each other in our work.Y ou ought not to (shouldn’t) write so carelessly.Oughtn’t we to (shouldn’t we) give him a chance to try?2. 表推测She ought to be home by now.Mr. Wang should know his address.3. should have done和ought to have done表虚拟,表示“本应该做而没做”,含责备的语气。
如:I should have closed the window yesterday.She ought to have been with the children.Y ou shouldn’t have left your daughter along.12.6 need/dare的用法这两个词即可作情态动词,又可作实义动词。
作情态动词没有过去时,往往用于否定或疑问句。
need表示需要、有必要,dare表示敢于。
如:We needn’t hurry.对need引起的疑问句,否定回答用needn’t,肯定回答用must。