知识点概述

一、情态动词的类型
1．只作情态动词用的有：must，can（could），may（might），ought to
2．可作情态动词也可作实义动词的有：need，dare（美语中常用作实义动词）
3．可作情态动词也可作助动词的有：shall（should），will（would）
4．具有情态动词的某些特征的有：have（had）to，used to

二、情态动词的语法特征
1．有一定的词义，但不能单独作谓语，它们要和行为动词或连系动词连用，构成谓语。
2．适用于主语的各种人称和数（have to例外，主语是第三人称单数时，要用has to），如：
　　We（He）must work hard. 我们（他）一定要努力工作。
　　I have to walk home. 我得步行回家。
　　He has to walk home. 他得步行回家。
3．后面接原形动词，即不带to的不定式（ought to, have to, used to本身带有to）如：
　　He may lose his way. 他可能迷路。
　　You ought to obey law. 你要遵守法律。
4．具有助动词的作用，即可用来构成否定句、疑问句及用于简明答语，如：
　　 I can't swim. Can you swim？（No, I can't, either.）我不会游泳，你会游泳吗？（我也不会。）

三、情态动词表可能和推测
1．can／could既可表能力（ability），又可表可能性（possibility），但may只能表示“可能性”不能表“能力”，be able to只能表能力；could只能表示过去一般性的能力，而was／were able to＋特定行为，则表示的确做成了某事，相当于managed to do sth，was／were successful in doing sth；succeeded in doing sth。表所将来才具备的能力只能用shall／will be able to．如：
　　 When he was three he could/was able to swim.（用于过去一般性的能力，二者皆可）
　　 His mother wastn't at home, so he was able to go to the film.
　　他母亲不在家，所以他去看电影了。（He could and did go. 他能够而且的确去了。只能够用was able to）
　　 The baby will be able to walk in a few weeks.（表示将来的能力，不能用can）
　　肯定句中，can可以表示客观的可能性，并不牵涉具体的事情是否会发生；要表示某事发生的实际可能性时，须用could, may或might。如：
　　 Accidents can happen on such rainy days.
　　这样的雨天，可能发生事故。
　　 Will you answer the doorbell？It may be your father.
　　请你开一下门好吗？可能是你父亲。（不用can）
　　 We may go camping next Sunday.
　　下星期天我们可能野营去。（can不能表示具体某事将会发生的可能性，要用may或might）
2．表示推测的情态动词层次比较

　注：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 can't be done by him. 这不可能是他干的。
　　This may not be done by him. 这可能不是他干的。
　　Can this be done by him? 这可能是他干的吗？（不可用 May，还可以说Is this likely to be done by him？）
　　This could（may／might ）be done by him.
　　这件事有可能是他干的。（语气依次递减）
　　He should be around sixty years old. 他可能60岁上下。
　　He ought to be home by now. 他现在该到家了。

四、could, might, would和should的委婉功能。
　　－Could / might I use your bike tomorrow morning？
　　－Yes，you can／may（否定：No, I'm aftaid not.）
　　不可说：Yes, you could／might 回答允许时，用could／might表委婉是不恰当的。
　　You are mistaken, I should say. 要我说，你是搞错了。
　　I should advise you not to do that. 我倒是劝你别这样做。
　　Would you please take a message for him？

五、情态动词可表示某一特定的语气或态度
1．can表示惊异、怀疑、不相信的态度（主要用于否定句、疑问句或感叹句中）。如：
　　This can't be done by him. 这不可能是他做的。
　　Can this be ture？这会是真的吗？
　　How can you be so careless！你怎么这么粗心！
2．may可以用于祈使句，表示祝愿。如：
　　May we never forget each other. 愿我们彼此水不相忘。
　　May you return in safety. 愿你平安归来。
3．Why／How＋should结构表示说话人对某事不能理解、感到意外、惊异等意思，译为“竟会”。如：
　　Why should you be so late today？你今天来得怎么这么晚？
　　I don't know why you should think that I did it. 我真不明白你凭什么认为这件事是我干的。
　　How should I know？我怎么会知道？（意为：我不知道。）

六、shall的用法要点：
1．用于第一、第三人称疑问句中，表示说话人征求对方的意见和向对方请示。如：
　　Shall the driver wait outside？让司机在外面等着吗？
　　When shall my father be able to leave hospital？我父亲什么时候可以出院？
2．用于第二、第三人称陈述句中，表示说话人给对方的命令、警告、允诺或威胁。如：
　　You shall go with me.（命令）
　　You shall have the book when I finish it.（允诺）
　　He should be punished.（威胁）

七、情态动词的一些惯用法：
1．cannot……too／enough表示“无论怎么……也不（过分）……”“越……越好”。如：
　　You cannot be too careful. 你越小心越好。
　　You cannot remember enough English words. 你记的英语单词越多越好。
2．cannot but＋do sth表示“不得不，只好”。
　　I cannot but admire your bravery. 我不得不钦佩你的勇气。
　　I could not but choose to go. 我只好去。
3．may well和may as well结构
　　“may well＋动词原形”是一种常用结构，意为“（完全）能，很可能，”相当于 to be very likely to。例如：
　　He may well be proud of his son. 他大可为儿子高兴。
　　Her appearance has changed so much that you may well not recognize her. 她的模样变化太大，你很可能认不出她了。
　　“may as well＋动词原形”意为“最好，满可以，倒不如”，相当于had better或 to have no strong reason not to。例如：
　　You may as well stay here over the night. 你最好留在这里过夜。
　　You may as well do it at once. 你最好马上就做这件事。
　　We may as well stay where we are. 我们留在现在的地方倒也不错。
　　You may as well tell me the truth. 你还是把真相告诉我的好。

八、“情动词＋have done”用法一览表

情态动词＋完成式	用法	例句
must have done	表示对过去已经发生的行为进行推测，意为“想必/准是/一定做了某事”。	It must have rained last night, for the road was quite muddy. They must have been asleep.
can ... have done cannot have done	表示对过去发生的行为的怀疑和不肯定，通常用在否定句和疑问句中。	He cannot have forgotten it. Can he have gone to his aunt's?
could have done	可用于肯定句中，表示“可能已经……”之意，此外，还可以表示过去没有实现的可能性，意为“本来可以……”。	You could have done better, but you were too careless. I saw Mr. Wang just now. He couldn't have gone to Beijing.
may / might have done	表示对过去已发生行为的推测，意为“也许/或许/已经（没有）…”。一般只用于肯定句或否定句中，不用于疑问句。用might则表示语气更加不肯定。	It's too late. I think he may have gone to bed. He may not have finished the work. She might have caught a cold.
might have done	表示“本来可能…”，但实际上没有发生的事。另外，还可以表示“本来应该或可以做某事”之意，含有轻微的责备语气。	You should not have swum in that sea. You might have been eaten by fish. You might have given him more help, though you were busy.
should / ought to have done	用于肯定句时，表示“本该做某事”而实际上未做；用于否定句时，则表示不该做的事反而做了。	You should have come to the meeting earlier. You ought to have done this exercise more carefully. You shouldn't have told her the truth. He ought not to have treated his parents like that.
needn't have done	表示做了本来不必去做的事，注意：didn't need to do 表示“没必要做而实际上也没有做某事”。	You needn't have taken a taxi here, for it was very near to my house. I didn't need to clean the windows. My brother did it.
had better have done	用于事后的建议，含轻微责备的口吻，意为“当时最好做了某事”，否定式had better not have done 表达相反的含义。	You had better have started earlier. You had better not have scolded her.
would rather have done	表示“当时宁愿做了某事”，其否定式 would rather not have done 表达相反含义，两者表示“后悔”之意。	I would rather have taken his advice. I raised objections at the meeting, but now I would rather not have done that.