情态动词和虚拟语气

1、情态动词和虚拟语气情态动词的用法要点一、 表能力CanAm/is/are able to 表现在的能力Will be able to 表将来的能力couldWas/were able to 表过去得能力Could have+过去分词：表示过去有能力做但未做二、 表推测（可能性）1. 可能性可分为客观的可能性和具体事情实际发生的可能性两种。所谓客观的（理论的）可能性即并不涉及具体某事是否会发生，此种用法常常可以说明人或事物的特征。情态动词can可用于肯定句中表示客观的（理论的）可能性，而表示具体事情实际发生的可能性时，can一般不用于肯定句。Accidents can happen on suc

2、h rainy days.We may go to the cinema tonight, but we are not sure yet.2. 表示具体事情实际发生的可能性：（1）表示具体事情实际发生的可能性的层次比较：词形肯定式否定式疑问式Must必定，必然/Will很可能，大概不会，不该会.吗Would可能性比will小语气比wont弱语气比will弱Should按说应该应该不会Ought to 按说应该应该不会Can不可能有可能吗Could微弱的可能不可能语气比can弱May 或许，也许，也未可知，也说不定可能不might比may还弱比may not 还弱（2）表示具体事情实际发生的可

3、能性时，may, must 常用于肯定句，一般不用于否定句和疑问句（may not 表示“可能不”）；can常用于否定句、疑问句或感叹句中，表示疑惑、惊讶或不相信等意思；could既可用于肯定句又可用于否定句、疑问句中。This cant/ couldnt be done by him.This may not be done by him.He could be on his way home now.Can this be done by him?Mr Mush is on time for everything. How can it be that he was late for th

4、e opening ceremony?I didnt hear the phone. I must have been asleep.-Are you coming to Jeffparty?-Im not sure. I might go to the concert instead.(3) would, could, should, might 并不一定与过去时间有关，而表示可能性弱于它们相应的现在时形式。This may/ might be done by him.(4)should 表推测时，表示确定或可能有的未来或期望，即合乎理想的情况或结果He ought to/ should b

5、e here on time-he started early enough.We should arrive before dark.The roads should be less crowded today.三、 表示请求、允许、允诺1. 当对方是决策者时，你代表你（们）自己（I, we），或代表第三者（he she they）向对方（you）请示或提出建议时用：（1） Shall/ May/ Might/ Can/ Could I/ we?（2） Shall he/ she/ they.?（3） Would/ Will you?Shall the driver wait outside

6、?When shall my father be able to leave hospital?Would you do me a favor?2. could/ might/ would/ should表委婉的语气-Could/ Might I use your bike tomorrow morning?-Yes, you can/ may.3. shall 还可以表命令、警告、威胁；宣布法律、规定时，也要用shallYou shall leave the room at once, and he also shall.It has been announced that candidat

7、es shall remain in their seats until all the papers have been collected.四、 表示必要性1. must 常用来表示必要性，意为“必须”（1） must+do sth. （2） Dont have to do sth./ dont need to do sth./ neednt do sth（3） 表示过去没必要做某事时，分为以下情况： neednt have done sth. Didnt have to do sth/ didnt need to do sth/ It was not necessary to do st

8、h.You neednt have worked that late last night. It was harmful to your health.-Did you work very late last night?-No, I didnt. I didnt need to work very late.Mustnt 意为“绝对不能，被禁止”You mustnt smoke here.2. should 也可以表示必要性，意为“（义务上）应该”。Should 表示必要性时，在语气上比must(必须)弱。“should +动词原形”，表示现在或将来应该做（的必要性）；“should+ h

9、ave+ 过去分词”，表示过去本该做（的必要性）I should go and visit him this afternoon, but I wonder if I will be free.You should have come to the conference yesterday. What was the reason for your absence?五、 表示“宁愿做某事”时，可使用下面这些句式：Would rather do sth.Would rather do sth.than sth.Would do sth. Rather than do sth.Would rath

10、er sb. Did sth.Prefer to do sth.Prefer to do sth. Rather than do sthPrefer doing sth. To doing sth.六、“情态动词+have done”用法一览表情态动词+have done用法例句Must have done想必/准是/一定做了否定形式为：Can/ could have done本来能够做过去可能会做Cannot/ could not have done过去不可能做了过去没能够做May/ might have done也许/或许已经做.， 一般只用于肯定句或否定句在（再否定句中表示“可能不”，不

11、用于疑问句中中。用might 则表示语气更加不肯定。Should/ ought to have done本该做，而实际上未做Neednt have done本不必做而做了Had better have done当时最好做了（其否定形式had better not have done表示相反的含义Would rather have done宁愿当时做了（其否定式would rather not have done表达相反含义，两者都表示“后悔”之意Would like/ love to have done sth.过去愿意做但未做成Shouldnt/ ought not to have done

12、本不该做而做了六、 情态动词的其他用法要点：1. cannot but +do sth.表示“不得不/只好做某事”I cannot but choose to go.2. may well 和may as well结构（1）“may well + 动词原形”是一种常用结构，意为“完全能，很可能”，相当于“be very likely to +动词原形”He may well be proud of his son.Her appearance has changed so much that you may well not recognize her.(2) “may as well + 动

13、词原形” 意为“最好，满可以，倒不如”You may as well do it at once.We may as well stay where we are.3.cannot (或can never 等)与enough 连用表示“再.也不为过”4.may 可以用于祈使句，表示祝愿May we never forget each other.May you return in safety.5.Why/ how + should结构表示说话人对某事不能理解，感到意外、惊讶等意思，意为“竟会”Why should you be so late today?How should I know?

14、6. must 表示“偏要、硬要“Must you make so much noise?If you must smoke, please go out.7. may 作“可以”讲时，其否定式常用mustnt 表示 “禁止”；must 作必须讲时，其否定式是neednt, 表示“不必”8. be able to 还有“设法做成”之意，相当于manage to do 或succeed in doingMany people were able to escape from the fire.9.need 和dare 的用法Need 和dare 两者既可用作情态动词也可用作行为动词。作行为动词时

15、，有人称和数的变化，可用于肯定句、否定句或疑问句中，构成否定句或疑问句时要用助动词do, does, did；作情态动词时，没有人称和数的变化（dare 的过去式为dared）,直接接动词原形，只用于否定句和疑问句中，构成否定句和疑问句时不用助动词。 另外，dare作情态动词时，还可用于条件句中，过去式为dared；作行为动词时，dare常用于疑问句或否定句，后面的to也可省略； Idare say 是习惯说法，意为“我想，大概” 虚拟语气一、 虚拟语气在非真实条件句中的运用从句主句例句与现在事实相反的假设If +主语+动词的过去式（be 动词用were）S + should/ would/c

16、ould/ might +动词原形与过去事实相反的假设If +主语+had + 过去分词S + should/would/might/could + have + 过去分词与将来事实相反的假设If +主语+动词的过去式If +主语+were to + 动词原形If + 主语+should + 动词原形S +should/would/could/might +动词原形使用虚拟条件句时要注意一下几点：1. 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 my advice

17、just now, you would be better now.If you had studied hard before, you would be a college student now.2. if 省略句在条件状语从句中可省略，把were, had, should 提到句首，变为倒装句式If I were at school again, I would study harder.If you had come earlier, you would have caught the bus.If it should rain tomorrow, we would not go c

18、limbing.3. 含蓄条件句有时为了表达的需要，在虚拟语气中并不总是出现if引导的条件状语从句，而是通过其他手段来代替条件句。What would you do with one million dollars?Without your help, we couldnt have finished the work ahead of time.I was ill that day. Otherwise, I would have taken part in the sports meeting.He telephoned to inform me of your birthday, or

19、I would have known nothing about it.4. even if, even though也可用于虚拟语气，其形式与if 从句在虚拟语气中的形式相同。二、 虚拟语气在名词性从句中的运用1. 用于宾语从句中（1） wish 后面的宾语从句一般用虚拟语气，表示一种不可能实现的愿望。其谓语动词构成形式为：过去式过去完成式Could/would +动词原形（2） 用于表示建议、愿望、命令等动词后的宾语从句中，常见的动词有：demand order require insist suggest recommend urge propose advise等。宾语从句中的虚拟语气结构为“should +动词原形”, should可以省略。He suggested that we should start off early the next day.They insisted that the boy s