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?Can you lift this heavy box?（體力） ?Mary can speak three languages.（知識）?Can you skate?（技能） 此時可用be able to代替。can只有一般現(xiàn)在時和一般過去式；而be able to則有更多的時態(tài)。 ?I’ll not be able to come this afternoon. 當(dāng)表示＂經(jīng)過努力才得以做成功某事＂時應(yīng)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. 此時可與may互換。在疑問句中還可用could, might代替，不是過去式，只是語氣更委婉，不能用于肯定句和答語中。?—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. 表示推測（驚訝、懷疑、不相信的態(tài)度），用于疑問句、否定句和感嘆句中。?This can’t be done by him. How can this be true?might比 may語氣更委婉，而不是過去式。否定回答時可用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...？征徇對方許可時比較正式和客氣，而用Can I...？在口語中更常見。might為may的過去式，但也可以代替may，語氣較為婉轉(zhuǎn)客氣或更加不肯定。He might not come today. 今天他也許不來了。(語氣不肯定) You might also get a headache when you work too hard. 當(dāng)你工作太努力時，你也可能患頭痛。在回答引出的問句時，如果是否定的，不能用mustn’t（禁止，不準(zhǔn)），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是說話人的主觀看法，而have to則強(qiáng)調(diào)客觀需要。must只有一般現(xiàn)在時， have to 有更多的時態(tài)形式。 ?I had to work when I was your age. 3. must表示推測、可能性（只用于肯定的陳述句）?You’re Tom’s good friend, so you must know what he likes best.?Your mother must be waiting for you now.①must開頭的疑問句，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? 我必須先完成作業(yè)嗎？—No, you don’t have to/ needn’t. 不，你不必。②can和must在表推測時，can一般用于否定句中，而must 常用于肯定句中。It can not be Li Lei. 那個人不可能是李磊。 It must be Li Lei. 那個人肯定是李磊。1. dare作情態(tài)動詞用時, 常用于疑問句、否定句和條件從句中, 過去式形式為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 作情態(tài)動詞用時, 常用于疑問句、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作實(shí)義動詞用時，有人稱、時態(tài)和數(shù)的變化。在肯定句中，dare后面常接帶to的不定式。在疑問句和否定句中，dare后面可接帶to或不帶to的不定式。而need后面只能接帶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、三人稱，表示說話人給對方的命令、警告、允諾或威脅。?You shall fail if you don’t work hard.(警告)?He shall have the book when I finish it.(允諾)?He shall be punished.(威脅)3. should表示義務(wù)、職責(zé)等，意為＂應(yīng)該＂。?You should be strict with yourself as an officer.作為一名官員，你應(yīng)該嚴(yán)格要求自己。?Will / Would you pass me the ball, please??I will never do that again. ?They asked him if he would go abroad. 3. would表示過去反復(fù)發(fā)生的動作或某種傾向。would表示過去習(xí)慣時比used to正式，且沒有＂現(xiàn)已無此習(xí)慣＂的含義。?During the vacation, he would visit me every other day.?The wound would not heal.1. should, ought to表示＂應(yīng)該＂，ought to表示義務(wù)或責(zé)任，比should語氣重。?I should help her because she is in trouble.?You ought to take care of the baby. 2. 表示勸告、建議和命令。should, ought to可通用，但在疑問句中常用should。?You should / ought to go to class right away.?Should I open the window?3. 表示推測 should , ought to (客觀推測), must(主觀推測)。?He must be home by now. (斷定他已到家) ?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)?表示對過去事情的肯定推測，譯成＂一定做過某事＂，該結(jié)構(gòu)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?yàn)榈孛孢€是濕的。?You must have been mad to speak to the servant. 你和仆人說話，一定是發(fā)瘋了。表示對過去事情的否定推測，譯成＂不可能做過某事＂。?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，我剛才還在圖書館見過他。?Mary can’t have stolen your money. She has gone home.?表示對過去行為的懷疑，用于疑問句，譯成＂可能做過……嗎？＂。?There is no light in the room. Can they have gone out??There is nowhere to find them. Where can they have gone?是虛擬語氣，表示對過去事情的假設(shè)，意思是本來能夠做某事而沒有做。?He could have passed the exam, but he was too careless. 本來他能夠通過考試，但是他太粗心。表示對發(fā)生過的事情的推測，意思是＂可能已經(jīng)＂或＂也許已經(jīng)＂，用于肯定句中。?—What has happened to George? 喬治發(fā)生了什么事？—I don’t know. He may have got lost. 我不知道，他可能迷路了。表示對過去事情的推測，might與may意思相同，但可能性更小。多用于虛擬語氣結(jié)構(gòu)中。?She might have achieved greater progress, if you had given her more chances.如果你多給她點(diǎn)機(jī)會，她可能已經(jīng)取得了更大的成績。虛擬語氣，表示對過去事情的假設(shè)，意思是＂本來會……＂。?I would have told you all about the boy’s story, but you didn’t ask me.意思是＂本來應(yīng)該做某事，而實(shí)際沒做＂。＂shouldn’t+have+done＂表示本來不應(yīng)該做某事，而實(shí)際做了。含有指責(zé)對方或自責(zé)的含義。?Tom, you are too lazy. The work should have been finished yesterday. 湯姆，你太懶惰了，這項(xiàng)工作本來應(yīng)該昨天就做完的。?Look, Tom is crying. I shouldn’t have been so harsh on him.看，湯姆哭了，我本不應(yīng)該對他如此嚴(yán)厲的。表示過去應(yīng)該做而實(shí)際并沒有做，譯成＂理應(yīng)做……＂，往往表示遺憾。與＂should have done＂用法基本一樣。?I ought to have gone home last Sunday. 我理應(yīng)上星期日回家。?You ought not to have given him more help. 你不應(yīng)該幫助他那么多。表示本來需要做某事而實(shí)際沒有做。＂needn’t have done＂則表示＂本來不需要做某事而實(shí)際做了＂?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. 他本來需要快點(diǎn)去車站的，那樣的話，他就不會誤了火車。(1) 從使用句型上看，can 通常只用于否定句或疑問句，一般不用于肯定句，而could 可用于肯定句、否定句和疑問句。兩者沒有時間上的差別，只是could 比 can 更委婉，更不確定。如：?It can’t [couldn’t] be true. 那不可能是真的。?What can [could] they be doing? 他們會在干什么呢??We could go there this summer. 今年夏天我們可能要去那兒。注：can 有時也用于肯定句中表示推測，主要用于表示理論上的可能性(即從理論上看是可能的，但實(shí)際未必會發(fā)生)，或表示＂有時＂之意。如：?Even experienced teachers can make mistakes. 即使是有經(jīng)驗(yàn)的教師也可能出錯。?She can be very unpleasant. 她有時很令人討厭。(2) 從時間關(guān)系看，對現(xiàn)在或?qū)砬闆r作推測，后接動詞原形；對正在進(jìn)行的情況作推測，后接 be doing 結(jié)構(gòu)；對過去情況作推測，后接動詞完成式。如：?He could have gone home. 他可能已經(jīng)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＂除表示對過去的推測外，還有以下重要用法：①表示過去沒有實(shí)現(xiàn)的可能性，常譯為＂本來可以＂。如：?I could have lent you the money. Why didn’t you ask me?②用來委婉地責(zé)備某人過去應(yīng)該做某事而沒有去做，常譯為＂本來應(yīng)該＂。如：?You could have helped him. 你本來應(yīng)該幫助他的。?I could have died laughing. 我差點(diǎn)兒笑死了。表示推測，兩者都可用，只是 might 比 may 語氣更不確定，表示的可能性更小。(1) 在句型使用方面：兩者均可用于肯定句和否定句，但用于疑問句時，may通常不用于句首，但可用于疑問句的句中（如特殊疑問句等），而might盡管可以用于疑問句的句首，但不算普通，通常會改用其他句式（如用could等）。如：?He may [might] know the answer. 他可能知道答案。?He may [might] not believe you. 他可能不會相信你。?And who may [might] she be? 那么她會是哪一位呢？(2) 從時間關(guān)系看，對現(xiàn)在或?qū)砬闆r作推測，后接動詞原形；對正在進(jìn)行的情況作推測，后接 be doing 結(jié)構(gòu)；對過去情況作推測，后接動詞完成式。如：?He may [might] tell his wife. 他也許會告訴他妻子。?He may [might] be writing a letter. 他可能在寫信。?She may [might] have read it in the papers. 她可能在報(bào)上已讀到過此事。(3)＂might+完成式＂除表示對過去的推測外，還有以下重要用法：① 表示過去某事可能發(fā)生而實(shí)際上卻并沒發(fā)生。如：?It was really very dangerous. I might have killed myself. 那真的是太危險(xiǎn)了，我差點(diǎn)沒命了。?A lot of men died who might have been saved. 很多人本來可以獲救的卻死了。?You might have made greater progress. 你的進(jìn)步本來可更大一些的。?You might at least have answered my letter. 你至少可以回我一封信嘛。must 表示很有把握的推測，其意為＂一定會＂＂肯定會＂，只用于肯定句，一般不用于否定句或疑問句。表示對現(xiàn)在或未來的情況作推測，后接動詞原形；表示對過去情況的推測，接動詞完成式。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，他一定在撒謊?He must have mistaken my meaning. 他一定誤會了我的意思。包含條件從句的句子稱為條件句。條件句分為兩類：一類是真實(shí)條件句，一類是虛擬條件句。如果假設(shè)的情況很可能發(fā)生，就用真實(shí)條件句。?If he has time, he wil come. 如果他有時間，他會來的。?He won’t succeed unless we plan well. 他不會成功的，除非我們計(jì)劃好。如果假設(shè)的情況發(fā)生的可能性不大，則用虛擬條件句。虛擬條件句分為三種，見下表：?If he had time now，he would(could，might)go with you.要是他現(xiàn)在有時間，他會和你一起去的。(與現(xiàn)在事實(shí)相反)?If you had come a few minutes earlier，you would(could，might) have met the famous singer.如果你早來幾分鐘，你就會見到那個著名歌手了。(與過去事實(shí)相反)?If it were to snow this evening，they would not go out.如果今天晚上下雪，他們就不會出去。(與將來事實(shí)相反)在if虛擬條件句中，若省略if，則將should，were，had前置，構(gòu)成主謂倒裝句式。?Had I worked harder，I would have succeeded.如果我當(dāng)時努力學(xué)習(xí)的話，我就會成功的。?Were she my daughter(Should she be my daughter)，I wouldn’t allow her to study abroad.如果她是我的女兒的話，我就不讓她出國學(xué)習(xí)了。若主句和從句所表示的時間不一致，就構(gòu)成了混合虛擬語氣或錯綜虛擬語氣，此時，主句和從句的時態(tài)形式根據(jù)實(shí)際的時間概念來定。?If she had followed the doctor’s words，she would feel better now.如果當(dāng)初她聽從了醫(yī)生的建議，現(xiàn)在就會好多了。有時候假設(shè)的條件不是通過條件從句表達(dá)，而是隱含在某些介詞短語中，如without...，連詞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)表示現(xiàn)在的情況，從句謂語用過去式；表示過去的情況，從句謂語用過去完成式；表示將來的情況，從句謂語用＂would/ should/could/might + 動詞原形＂。?I wish I were ten years younger now. 我希望我現(xiàn)在年輕十歲。?I wish I had met him yesterday. 我昨天能遇到他就好了。?I wish I would be a scientist. 我希望將來成為科學(xué)家。2. 在表示請求、建議、命令、要求等動詞后的賓語從句中，謂語用＂should + 動詞原形＂，should可省略常見的這類動詞有：suggest，recommend， insist， order， require， advise， demand， command， ask， request，propose等。?He suggested that we (should) be here in time. 他建議我們及時到那兒。?The doctor advised that he (should) change his job. 醫(yī)生勸他換工作。3. 在would rather后的賓語從句中，謂語常用過去式來表示現(xiàn)在或?qū)淼那闆r，用過去完成式表示過去的情況。?I would rather you told me the truth. 我愿意你跟我講真話。?I would rather you had been present. 我倒愿意你當(dāng)時在場。4. 在＂It is (about/high) time＋that從句＂中，謂語動詞常用過去式或＂should＋動詞原形＂表示虛擬語氣，注意should不可省略。?It’s high time that we devoted ourselves to environmental protection.是我們致力于環(huán)境保護(hù)的時候了。5. as if，as though引導(dǎo)的表語從句和方式狀語從句中常用虛擬語氣，與現(xiàn)在事實(shí)相反用一般過去時；與過去事實(shí)相反用過去完成時。?They talked and talked as if they would never meet again.6. if only引導(dǎo)的感嘆句中，用虛擬語氣表示愿望。?Look at the trouble we’re in. If only we had taken our teacher’s advice！看看我們所處的困境，要是我們接受老師的建議就好了！當(dāng)條件狀語從句表示的行為和主句表示的行為所發(fā)生的時間不一致時，被稱為＂錯綜時間條件句＂，動詞的形式要根據(jù)它所表示的時間做出相應(yīng)的調(diào)整。?If you had followed the doctor’s advice, you would be better now.如果你當(dāng)時聽醫(yī)生建議的話，你現(xiàn)在就好多了。
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