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1、1情态动词和虚拟语气【知识点归纳】I.情态动词1.表示“能力、许 可”的 can /may表示能力的情态动词用 can/couldA computer _think for itself, it must be told what to do. A. cant B. couldnt C. may not D. might not表示 许可时 用 may/might ,can/could 都可以，但在 问句中用 could? 或 might? 以使口气委婉客气，其回答一定要用 can 或 may，以使回答口气明确(must 表示一 定，必须，mustnt 表示禁止，不许可)Tom, you _
2、play with the knife; you _ hurt yourself. A. wont, cant B. mustnt/may C. shouldnt/must D. cant, shouldnt在肯定句中 could 不可以用来表示过去某一特定场合的能力，而要用 was/were able to。例如：they were able to/managed to jump into但在否定句中 could/was(were)able to 可以通用: I couldnt/wasnt able to see him yesterday.表示许可时用 may/can 都可以。只不 过用
3、may 时较侧 重讲话人的许可，而 can 较侧重客观情况的许可， must 则表示必须做某事。2.表示“推断、判断 ”的 can, may, must在肯定句中都可以用来表示可能。在含义上 must 语气最为肯定,may 表示的是事实上的可能性：Peter _ come with us tonight, but he isnt very sure yet. A. must B. may C. can D. will而 can 表示的是逻辑上的可能性：Mary is in poor health. She can be ill at any times.在否定句中只能用 can 和 may。此
4、时 cant 用以代替 mustnt，语气比 may not 更强。Cant 中文可以翻译为“不可能 ”、may not 中文可以翻 译为“可能不”。Michael_ be a policeman, for hes much too short. A. neednt B. cant C. should D. may在疑 问句只能用 can，不能用 may 和 mustHe may be very busy now. Can he be very busy now? He must be very busy now.3.表示“请求、提 议”(用在疑问 句中) 的 can 和 could，这时 c
5、ould 比 can 语气更婉转Can (Could)you give me a lift to the station?4.表示“惊讶、怀 疑”(用在疑问 句中) 的 can 和 could，这时 could 比 can 语气更婉转Who can it be at this time of day?这个时候到底会是谁呢?5.构成下列特殊句式的 cancan not/can nevertoo cannotenough“无论怎么也不为过分，越越好，非常”cant help doing, cannot help but do, cannot but do“禁不住，不由得，不得不”6.表示祝愿、祝福
6、的 may,此时句子用倒装语序 May he live to an old age!愿他长命百岁!7.shall: 用于第一、三人称表征求对方的意见; 用于第二、三人称表 说话人给对方的警告、命令、允 诺、威 胁；用于法律、法规等条文中表 应，须 ，得 What shall we do this evening? 注意：may 用于征求对方的许可，而 shall 用于征求对方的意见或指示：May I have a look?我能看一看吗? Shall I have a look? 需要我看一看吗?You shall fail if you dont work harder. (警告) He s
7、hall have the book when I finish reading. (允诺)He shall be punished.(威胁) The fine shall be given in cash.罚款须以现金缴纳。8. should （表义务 、责任的）应该 You should apologize to him（表预 期的） 应该会，想必会，一定会把 The photos should be ready by 12:00.（表惊 讶、 遗憾的）竟然、居然 Im surprised that you should speak in such a way.（与疑 问词连 用，表意外、
8、惊讶、 纳闷等的）究竟是，到底 Who should come in but my old friend Betty.我当究竟是谁进来了呢，原来是我的老朋友贝蒂呀。（用于表示命令、建议、请求等 词后面的 that 从句中，且 should 可以省略）应该，必 须I suggested that he (should)change his mind. My suggestion was that he (should)change his mind.注意：ought to 表示 “应该” (与 should 同义, 只是语气稍重一些), 也可表示推测。 2You ought to take ca
9、re of him. He ought to be home by now.9.will/would 请求、建议，would 比 will 委婉客气 Would you pass me the book?表示意志、愿望和决心 I will never do that again. They asked us if we would do that again可表示 过去反复 发生的动作或某种倾向 During the vacation he would visit me every week表示估 计或猜想 It would be about ten when he left home. Wh
10、at would she be doing there?10.dare 和 need need 作为情态动词只有一种形式，只用于否定句和疑问句。 dare 作为情 态动词用时有两中形式： dare 和 dared 两个词 形。 除了可以用于否定句和疑问句外，还可以用于条件从句或表示怀疑的句子中。If he dare come, I will kick him out. I dont know whether he dare say.10.情态动词+have done 的用法 :could + have done：本可以做而实际上未能做 You could have done better, b
11、ut you didnt try your best.cannot + have done：表示对过去行为的否定推测 He cannot have been to that town.can+主语+have done ：表示对过去行为的怀疑或不肯定 Can he have got the book?might(may)+ have done：对过去发生的行为不太肯定的推测 He may not have finished the work.must + have done：对过去发生的行为肯定的推测。其否定式为：cannot have doneYou must have seen the fi
12、lm. You cannot have seen the film.neednt + have done：本来不必要做的而实际上又做了 You neednt have watered the flowers, for it is going to rain.注意：didnt need to(have to)do :没有必要做而实际上也没有做 I didnt need to clean the windows. Mary did it 2 hours ago.should(ought to)+ have done：本来应该做而实际上又没有做。 其否定形式表示某种行为不该发生却发生了。You sh
13、ould have started earlier, but you didnt. She shouldnt have taken away my measuring tape, for I wanted to use it.注意：本结构还可以表“推测”的意思 He should have finished the work by now.11.其它情态动词have to 表示 “必须, 不得不”, 在这个意义上与 must 很接近, 但 must 表示的是说话人的主观看法, 而 have to 表示的却是客观需要. have to 比 must 有更多的形式。 注意：在回答 must 的问句
14、时, 否定式常用 need not(neednt)或 dont have to 表示 “不必”。而不用 must not, 因为 must not 表示 “不可以”。used to表示过去的习惯动作或状态(现在已不复如此)Did you use to go there to see your brother? /Used you to go there to see your brother?Yes, I did(used to). /No , I didnt (usednt).注意：would 也可表示过去反复发生的动作或某种倾向，但是不涉及到现在。had better 表示 “最好(做)”
15、 其否定式为 had better notII.虚拟语气(一)一般虚拟结构类别 用法 例句与现在事实相反 从句动词：过去式（be 用 were）;主句动词：should/would/could/might+动词原形 If he were here, he would help us.与过去事实相反 从句动词：had+过去分词主句动词：should/would/could/might + have+过去分词 If I had been free, I would have visited you.If引导的条件从句与将来事实相反 从句动词：过去式 / should+ 动词原形 / were+ 不定式主句动词：should/would/could/might+ 动词原形 If it should rain tomorrow, we would not go camping.注意:主句中的 should 只用于 I、we，但在美国英 语中， should 常被 would 代替;从句中的 should 可用于各种人称，且不可以被 would 所代替。3(二)混合虚拟结构: 1.不同 时间的虚拟If he had listened to me, h
