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☞Can you lift this heavy box?（体力） ☞Mary can speak three languages.（知识）☞Can you skate?（技能） 此时可用be able to代替。can只有一般现在时和一般过去式；而be able to则有更多的时态。 ☞I’ll not be able to come this afternoon. 当表示＂经过努力才得以做成功某事＂时应用be able to，不能用can。如：☞He was able to go to the party yesterday evening in spite of the heavy rain.—Yes, you can. / No, you can’t. 此时可与may互换。在疑问句中还可用could, might代替，不是过去式，只是语气更委婉，不能用于肯定句和答语中。☞—Could I come to see you tomorrow?—Yes, you can. (No, I’m afraid not.)☞They’ve changed the timetable, so we can go by bus instead. This hall can hold 500 people at least. 4. 表示推测（惊讶、怀疑、不相信的态度），用于疑问句、否定句和感叹句中。☞This can’t be done by him. How can this be true?might比 may语气更委婉，而不是过去式。否定回答时可用can’t 或mustn’t，表示＂不可以，禁止＂。☞——Might/ May I smoke in this room?☞——May/Might I take this book out of the room?——Yes, you can. (No, you can’t / mustn’t. ) 用May I...？征徇对方许可时比较正式和客气，而用Can I...？在口语中更常见。might为may的过去式，但也可以代替may，语气较为婉转客气或更加不肯定。He might not come today. 今天他也许不来了。(语气不肯定) You might also get a headache when you work too hard. 当你工作太努力时，你也可能患头痛。在回答引出的问句时，如果是否定的，不能用mustn’t（禁止，不准），而用needn’t, don’t have to（不必）。☞——Must we hand in our exercise books today?——Yes, you must. /No, you don’t have to / you needn’t. 2. must是说话人的主观看法，而have to则强调客观需要。must只有一般现在时， have to 有更多的时态形式。 ☞I had to work when I was your age. 3. must表示推测、可能性（只用于肯定的陈述句）☞You’re Tom’s good friend, so you must know what he likes best.☞Your mother must be waiting for you now.①must开头的疑问句，其否定回答通常用don’t have to或needn’t。而不用mustn’t。 —Must I finish my homework first? 我必须先完成作业吗？—No, you don’t have to/ needn’t. 不，你不必。②can和must在表推测时，can一般用于否定句中，而must 常用于肯定句中。It can not be Li Lei. 那个人不可能是李磊。 It must be Li Lei. 那个人肯定是李磊。1. dare作情态动词用时, 常用于疑问句、否定句和条件从句中, 过去式形式为dared。☞How dare you say I’m unfair? ☞He daren’t speak English before such a crowd, dare he? ☞If we dared not go there that day，we couldn’t get the beautiful flowers. 2. need 作情态动词用时, 常用于疑问句、否定句。在肯定句中一般用must, have to, ought to, should代替。 ☞You needn’t come so early. ☞—Need I finish the work today? —Yes, you must. / No, you needn’t. dare和 need作实义动词用时，有人称、时态和数的变化。在肯定句中，dare后面常接带to的不定式。在疑问句和否定句中，dare后面可接带to或不带to的不定式。而need后面只能接带to的不定式。 ☞I dare to swim across this river.☞He doesn’t dare (to) answer. ☞He needs to finish his homework today.☞What shall we do this evening? 2. shall 用于第二、三人称，表示说话人给对方的命令、警告、允诺或威胁。☞You shall fail if you don’t work hard.(警告)☞He shall have the book when I finish it.(允诺)☞He shall be punished.(威胁)3. should表示义务、职责等，意为＂应该＂。☞You should be strict with yourself as an officer.☞Will / Would you pass me the ball, please?☞I will never do that again. ☞They asked him if he would go abroad. 3. would表示过去反复发生的动作或某种倾向。would表示过去习惯时比used to正式，且没有＂现已无此习惯＂的含义。☞During the vacation, he would visit me every other day.☞The wound would not heal.1. should, ought to表示＂应该＂，ought to表示义务或责任，比should语气重。☞I should help her because she is in trouble.☞You ought to take care of the baby. 2. 表示劝告、建议和命令。should, ought to可通用，但在疑问句中常用should。☞You should / ought to go to class right away.☞Should I open the window?3. 表示推测 should , ought to (客观推测), must(主观推测)。☞He must be home by now. (断定他已到家) ☞He ought to/should be home by now.(不太肯定)☞This is where the oil must be.(直爽) ☞This is where the oil ought to/should be.(含蓄)☞表示对过去事情的肯定推测，译成＂一定做过某事＂，该结构只用于肯定句。☞It must have rained last night, for the ground is wet. 昨晚一定下雨了，因为地面还是湿的。☞You must have been mad to speak to the servant. 你和仆人说话，一定是发疯了。表示对过去事情的否定推测，译成＂不可能做过某事＂。☞Mr Smith can’t have gone to Beijing, for I saw him in the library just now.史密斯先生不可能去北京了，我刚才还在图书馆见过他。☞Mary can’t have stolen your money. She has gone home.☞表示对过去行为的怀疑，用于疑问句，译成＂可能做过……吗？＂。☞There is no light in the room. Can they have gone out?☞There is nowhere to find them. Where can they have gone?是虚拟语气，表示对过去事情的假设，意思是本来能够做某事而没有做。☞He could have passed the exam, but he was too careless. 本来他能够通过考试，但是他太粗心。表示对发生过的事情的推测，意思是＂可能已经＂或＂也许已经＂，用于肯定句中。☞—What has happened to George? 乔治发生了什么事？—I don’t know. He may have got lost. 我不知道，他可能迷路了。表示对过去事情的推测，might与may意思相同，但可能性更小。多用于虚拟语气结构中。☞She might have achieved greater progress, if you had given her more chances.虚拟语气，表示对过去事情的假设，意思是＂本来会……＂。☞I would have told you all about the boy’s story, but you didn’t ask me.意思是＂本来应该做某事，而实际没做＂。＂shouldn’t+have+done＂表示本来不应该做某事，而实际做了。含有指责对方或自责的含义。☞Tom, you are too lazy. The work should have been finished yesterday. ☞Look, Tom is crying. I shouldn’t have been so harsh on him.表示过去应该做而实际并没有做，译成＂理应做……＂，往往表示遗憾。与＂should have done＂用法基本一样。☞I ought to have gone home last Sunday. 我理应上星期日回家。☞You ought not to have given him more help. 你不应该帮助他那么多。表示本来需要做某事而实际没有做。＂needn’t have done＂则表示＂本来不需要做某事而实际做了＂☞I needn’t have bought so much wine — only five people came.☞He need have hurried to the station. In that case, he wouldn’t have missed the train. 他本来需要快点去车站的，那样的话，他就不会误了火车。(1) 从使用句型上看，can 通常只用于否定句或疑问句，一般不用于肯定句，而could 可用于肯定句、否定句和疑问句。两者没有时间上的差别，只是could 比 can 更委婉，更不确定。如：☞It can’t [couldn’t] be true. 那不可能是真的。☞What can [could] they be doing? 他们会在干什么呢?☞We could go there this summer. 今年夏天我们可能要去那儿。注：can 有时也用于肯定句中表示推测，主要用于表示理论上的可能性(即从理论上看是可能的，但实际未必会发生)，或表示＂有时＂之意。如：☞Even experienced teachers can make mistakes. 即使是有经验的教师也可能出错。☞She can be very unpleasant. 她有时很令人讨厌。(2) 从时间关系看，对现在或将来情况作推测，后接动词原形；对正在进行的情况作推测，后接 be doing 结构；对过去情况作推测，后接动词完成式。如：☞He could have gone home. 他可能已经回家了。☞He can’t [couldn’t] have understood. 他不可能理解了。☞Why does he know this? Can [Could] someone have told him about it?(3) ＂could+完成式＂除表示对过去的推测外，还有以下重要用法：①表示过去没有实现的可能性，常译为＂本来可以＂。如：☞I could have lent you the money. Why didn’t you ask me?②用来委婉地责备某人过去应该做某事而没有去做，常译为＂本来应该＂。如：☞You could have helped him. 你本来应该帮助他的。☞I could have died laughing. 我差点儿笑死了。表示推测，两者都可用，只是 might 比 may 语气更不确定，表示的可能性更小。(1) 在句型使用方面：两者均可用于肯定句和否定句，但用于疑问句时，may通常不用于句首，但可用于疑问句的句中（如特殊疑问句等），而might尽管可以用于疑问句的句首，但不算普通，通常会改用其他句式（如用could等）。如：☞He may [might] know the answer. 他可能知道答案。☞He may [might] not believe you. 他可能不会相信你。☞And who may [might] she be? 那么她会是哪一位呢？(2) 从时间关系看，对现在或将来情况作推测，后接动词原形；对正在进行的情况作推测，后接 be doing 结构；对过去情况作推测，后接动词完成式。如：☞He may [might] tell his wife. 他也许会告诉他妻子。☞He may [might] be writing a letter. 他可能在写信。☞She may [might] have read it in the papers. 她可能在报上已读到过此事。(3)＂might+完成式＂除表示对过去的推测外，还有以下重要用法：☞It was really very dangerous. I might have killed myself. 那真的是太危险了，我差点没命了。☞A lot of men died who might have been saved. 很多人本来可以获救的却死了。☞You might have made greater progress. 你的进步本来可更大一些的。☞You might at least have answered my letter. 你至少可以回我一封信嘛。must 表示很有把握的推测，其意为＂一定会＂＂肯定会＂，只用于肯定句，一般不用于否定句或疑问句。表示对现在或未来的情况作推测，后接动词原形；表示对过去情况的推测，接动词完成式。如：☞I must be the happiest woman on earth! 我一定是世界上最幸福的女人了。☞No, he must be lying. 不，他一定在撒谎☞He must have mistaken my meaning. 他一定误会了我的意思。包含条件从句的句子称为条件句。条件句分为两类：一类是真实条件句，一类是虚拟条件句。☞If he has time, he wil come. 如果他有时间，他会来的。☞He won’t succeed unless we plan well. 他不会成功的，除非我们计划好。如果假设的情况发生的可能性不大，则用虚拟条件句。虚拟条件句分为三种，见下表：☞If he had time now，he would(could，might)go with you.要是他现在有时间，他会和你一起去的。(与现在事实相反)☞If you had come a few minutes earlier，you would(could，might) have met the famous singer.如果你早来几分钟，你就会见到那个著名歌手了。(与过去事实相反)☞If it were to snow this evening，they would not go out.如果今天晚上下雪，他们就不会出去。(与将来事实相反)在if虚拟条件句中，若省略if，则将should，were，had前置，构成主谓倒装句式。☞Had I worked harder，I would have succeeded.☞Were she my daughter(Should she be my daughter)，I wouldn’t allow her to study abroad.若主句和从句所表示的时间不一致，就构成了混合虚拟语气或错综虚拟语气，此时，主句和从句的时态形式根据实际的时间概念来定。☞If she had followed the doctor’s words，she would feel better now.有时候假设的条件不是通过条件从句表达，而是隐含在某些介词短语中，如without...，连词but for，otherwise，or等中。☞Without music(＝If there were no music)，the world would be dull.☞I’m really very busy，otherwise I would certainly go there with you.(otherwise＝if I were not so busy)表示现在的情况，从句谓语用过去式；表示过去的情况，从句谓语用过去完成式；表示将来的情况，从句谓语用＂would/ should/could/might + 动词原形＂。☞I wish I were ten years younger now. 我希望我现在年轻十岁。☞I wish I had met him yesterday. 我昨天能遇到他就好了。☞I wish I would be a scientist. 我希望将来成为科学家。2. 在表示请求、建议、命令、要求等动词后的宾语从句中，谓语用＂should + 动词原形＂，should可省略常见的这类动词有：suggest，recommend， insist， order， require， advise， demand， command， ask， request，propose等。☞He suggested that we (should) be here in time. 他建议我们及时到那儿。☞The doctor advised that he (should) change his job. 医生劝他换工作。3. 在would rather后的宾语从句中，谓语常用过去式来表示现在或将来的情况，用过去完成式表示过去的情况。☞I would rather you told me the truth. 我愿意你跟我讲真话。☞I would rather you had been present. 我倒愿意你当时在场。4. 在＂It is (about/high) time＋that从句＂中，谓语动词常用过去式或＂should＋动词原形＂表示虚拟语气，注意should不可省略。☞It’s high time that we devoted ourselves to environmental protection.5. as if，as though引导的表语从句和方式状语从句中常用虚拟语气，与现在事实相反用一般过去时；与过去事实相反用过去完成时。☞They talked and talked as if they would never meet again.6. if only引导的感叹句中，用虚拟语气表示愿望。☞Look at the trouble we’re in. If only we had taken our teacher’s advice！看看我们所处的困境，要是我们接受老师的建议就好了！当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，被称为＂错综时间条件句＂，动词的形式要根据它所表示的时间做出相应的调整。☞If you had followed the doctor’s advice, you would be better now.
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