概 说
助动词(auxiliary)主要有两类：基本助动词(primary auxiliary)和情态助动词(modal auxiliary)。基本助动词有三个：do, have和be；情态助动词有十三个：may, might； can, could； will, would； shall, should； must, need, dare, used to, ought to。 上述两类助动词的共同特征是，在协助主动词构成限定动词词组时，具有作用词的功能：
1）构成否定式：
He didn’t go and neither did she.
The meeting might not start until 5 o’clock.
2）构成疑问式或附加疑问式： Shall I write to him?
You have been learning French for 5 years, haven’t you?
3）构成修辞倒装：
Nowhere can he obtain any information about his sister. Hardly had he arrived when she started complaining.
4）代替限定动词词组：
A: Who can solve this crossword puzzle? B: Tom can.
A: Must you leave right now? B: Yes, I must.
一、情态动词的主要特征
试比较下列几组句子，看情态动词的特点：
1、He borrows some books from the library. 他常从图书馆借些书。 He can borrow some books from the library. 他可以从图书馆借到一些书。
2、I run fast. 我跑得快。
I can’t run fast. 我跑得不快。 can’t 不会，不能
3、Must he go now? 他必须现在走吗？ must 必须 Did he go last night? 他昨晚去了吗？
4、They may not be there. 他们可能不在那儿。 may 可能 They weren’t there. 他们不在那儿。
从上述四组句子中，可以看出以下几个特点：
1、can、may、must都有各自的词义，表示能力、可能、允诺、愿意、请求等情态，因词义不完全，不能单独作谓语，只能和动词原形一起构成谓语。
2、情态动词没有人称和数的变化。构成疑问句时，通常放在主语前面（见例3）；构成否定句时，not放在这些情态动词之后（见例2）。可用它们的缩写形式（can’t, cannot, mustn’t等）。
现将情态助动词的各种形式列表如下：
肯定式 may might can could will, 'll would, 'd shall should must need dare used to ought to 二、具体用法
（一）can, could 和 be able to 1、表示能力，例如： I can speak a little Japanese. 我会说一点儿日语。
She couldn’t speak Chinese when she came to our school last month. 上月她来我校时还不会说中文。
be able to代替can, 也可以表示能力。但can只有一般现在时和一般过去时(could), 而be able to则有更多的时态形式，例如：
You will be able to talk with the foreign teacher in English next week. 下星期你将能与外国老师用英语交谈了。 My little brother has been able to write. 我的小弟弟已经会写字了。
2、表示允许，准许，这时can与may可以互换，例如： Can/May I borrow your bike tomorrow?
否定式 may not might not can not, cannot could not will not, 'll not would not, 'd not shall not should not must not need not dare not used not to ought not to 简略否定式 mayn't mightn't can't couldn't won't wouldn't shan't shouldn't mustn't needn't daren't didn't use(d) to usedn't to oughtn't to 明天我可以借你的自行车吗？
Yes, of course. You can/may use my bike tomorrow. 当然可以。明天你可以用我的自行车。 You can’t smoke here. 你不可以在这儿抽烟。
3、表示客观可能性，用在否定句和疑问句中表示说话人的怀疑、猜测或不肯定。例如： He cannot/can’t be there. 他不可能在那儿。 Can this news be true? 这消息可能真实吗？
4、could 除表示can的过去式外，在口语中还常代替can，表示非常委婉的请求。这时could和can没有时间上的差别。例如：
Could/Can you tell me if he will go tomorrow? 你能告诉我他明天是否去吗？
Could/Can I ask you something if you are not busy? 如果您不太忙，我能否问您一些事情？
Could/Can you show me the way to the nearest hospital? 您能给我指一下去最近的医院的路吗？
（二）may 和 might
1、表示“准许”和“许可”，这时可与can替换。例如： May (can) I use your dictionary for a moment? 我可以借你的字典用一下吗？
May I take these magazines out of the reading room? No, you mustn’t. 我可以把这些杂志带出阅览室吗？ 不，不行。 He asked me if he might go then. 他问我他是否可以走了。
2、表示说话人的猜测，认为某事“可能”发生，例如： Where’s John? He may be at the library. 约翰在哪儿？他可能在图书馆。
Mr. Green hasn’t talked with her. He may not know her. 格林先生还未曾与她谈过话，他可能不认识她。
以上例子中的may be是情态动词may加be, 与maybe完全不同。后者是副词，解释为“或许”。例如： He may be at home. 他可能在家。 Maybe he was at home. 或许他在家。
3、might除表示may的过去式外，在口语中还常代替may, 表示非常委婉的请示或实现的可能性较小。这时might和may没有时间上的差异。例如： Might (May) I speak to you for a few minutes? 我现在可以与你谈几分钟话吗？ Might I have a photo of your family? 我可以要一张你们的合家照吗？
4、用于从句中表示目的，意为“以便能……”、“使……可以”，例如： Open your mouth wide, so that I may see clearly what’s wrong with your teeth. 把嘴张大些，以便我能看清楚你的牙齿有什么毛病。 He wrote down my address so that he might remember it well. 他把我的住址写了下来，以便能记牢。
5、在用may提问时，否定回答常用mustn’t或may not表示“不行”、“不可以”。例如： May I go now? No, you mustn’t. 我可以走了吗？ 不，不可以。
（三）must
1、must表示说话人的主观意志，表示义务、命令或必要、应当和必须等。现在式与过去式同形。例如：
I must go to school today. 今天我必须上学去。
He told me I mustn’t leave until my mother came. 他告诉我，在我母亲回来之前我不许离开。
2、must表示推测，“一定是”、“准是”，例如： They must be very tired. Let them have a rest. 他们一定是非常疲劳了。让他们休息一会儿吧。 Jack doesn’t look well. He must be ill. 杰克看上去气色不太好。他一定是病了。
[难点解释]
3、have to 表示“必须”、“不得不”，它不仅能代替must, 用于现在时和过去时以外的其他时态，表示说话人的主观看法，而且又表示客观上的需要。例如： If we miss the last bus, we shall have to walk home. 如果我们末班车，我们将不得不走回家。
The ship started to go down slowly. We must leave the ship. 船慢慢地开始下沉了。我们必须离开这船。
注意：
1） 用have to表示客观上的需要，意为“不得不”、“只是”；must则表示主观上的必须和说话人的意志
2） 有些时态用must无法表示，可用have to代替，如will have to（表示将来），had to（表
示过去）。
4、在回答must的疑问句时，否定回答常用needn’t表示“不必”，例如： Must I return this book to you in two weeks? 这本书我两星期以后必须还你吗？ Yes, you must. 是的。 No, you needn’t. 不，不必了。
（四）need 和 dare need（需要）和dare（敢于）既可作情态动词，又可作行为动词。 1、need和dare作情态动词时，只用于否定句或疑问句。need无形态变化，dare的过去式是dared。 例如：
It’s warm today. You needn’t put on your coat. 今天天气很暖和，你不必穿上大衣。 Need I post your books to you? 要我把书寄给你吗？ How dare you say it’s unfair? 你怎么胆敢说这不公平呢？ She dare not go out alone at night. 她晚上不敢一个人出去。
2、need和dare作及物动词时，后常跟动词不定式。它们有人称、数和时态等形态变化。在构成否定和疑问形式时与其他及物动词一样，要用助动词do, does或did等。例如： He didn’t need to go to school today. 今天他不必去上学。
They needed an excuse and soon found one. 他们需要借口，不久便找到了一个。
（五）ought to 和 should
ought to和should作情态度动词用，都是“应该”、“应当”的意思。ought to语气较强，指客观上有责任、有义务去做某事，或按观念和道理也应对某事负责。Should指主观上认为有责任和义务去做，但语气不如ought to强。例如： You ought to respect your teachers. 你们应该尊敬你们的老师。
We should be careful of others’ feelings. 我们应该尊重别人的感情。
（六）相当于情态动词的几个固定词组
had better...（最好……），Shall I（we）...？（我/我们可以这样做吗？）would like...（非常想……），Will/Would you (please)...？（请你……吗？）used to...（过去常常……）。例如： It’s late. I’d better go and look for him. 太迟了。我最好去找他。
You’d better not read books in poor light. 你最好不要在微弱的灯光下看书。
Shall we start the meeting at once? 我们立即开会好吗？ Will you get me some chalk? 你拿些粉笔给我好吗？ Would you like some bananas? 来点香蕉好吗？
三. 巩固练习
1. I didn’t see her in the meeting room this morning. She _____ at the meeting. A. mustn’t have spoken B. shouldn’t have spoken C. needn’t have spoken D. couldn’t have spoken
2. One ought _____ for what one hasn’t done.
A. not to be punished B. to not be punished C. to not punished D. not be punished
3. If you really want yourself to be in good health, you must ___ always ___ so much. A. not; be smoking B. not; have smoked C. not; smoke D. be not; smoking
4. With so much work on hand, you _____ to see the game last night.
A. mustn’t go B. shouldn’t go C. couldn’t have gone D. shouldn’t have gone
5. Most of the students felt rather disappointed at the English party. They say that it ______ better organized.
A. had been B. had to be C. must have been D. could have been
6. I’m surprised that he _____ in the exam.
A. should fail B. would have failed C. may have failed D. should have failed
7. The little girl _____ there alone.
A. not dare go B. dares not go C. dare not go D. dare not to go
8. “Must we do it now?” “No, you _____.”
A. won’t B. needn’t C. can’t D. don’t
9. He said he would rather not _____ it right now.
A. doing B. to do C. do D. to be doing
10. You _____ to the meeting this afternoon if you have something important to do. A. needn’t to come B. don’t need come C. don’t need coming D. needn’t come
11. Put on more clothes. You _____ be feeling cold with only a shirt on.
A. can B. could C. would D. must
12. I _____ play football than baseball.
A. would rather B. had better C. like better D. prefer
13. I thought you _____ like something to read, so I have brought you some books. A. may B. might C. could D. must
14. There was plenty of times. She _____.
A. mustn’t have hurried B. couldn’t have hurried C. must not hurry D. needn’t have hurried
15. The plant is dead. I _____ it more water.
A. will give B. would have given C. must give D. should have given
16. You _____ return the book now. You can keep it till next week if you like. A. can’t B. mustn’t C. needn’t D. may not
17. It’s still early, you _____.
A. mustn’t hurry B. wouldn’t hurry C. may not hurry D. don’t have to hurry
18. Please open the window, _____?
A. can’t you B. aren’t you C. do you D. will you
19. We _____ for her because she never came.
A. mustn’t have waited B. shouldn’t have waited C. mustn’t wait D. needn’t wait
20. --- May I stop here? --- No, you _____.
A. mustn’t B. might not C. needn’t D. won’t
21. It’s a fine day. Let’s go fishing, _____.
A. won’t we B. will we C. don’t we D. shall we
22. Everyone _____ do his best for the modernizations of our country. A. can B. may C. should D. might
23. --- Please don’t make a noise. --- _____. I’ll be as quiet as a mouse.
A. Yes, I won’t B. No, I won’t C. No, I will D. Yes, I will
24. The young man has made so much noise that he _____ not have been allowed to attend
the concert.
A. could B. must C. would D. should
25. --- Where is John? --- He _____ in the library.
A. should be B. must be C. can be D. must have been
26. Since the road is wet this morning, _____ last night. A. it must rain B. it must be raining C. it must have rained D. it must have been rain
27. — Will your brother stay home tonight?
— I’m not quite sure. He _____ to the cinema tonight.
A. must go B. can go C. may go D. may be going
28. He _____ have come here yesterday, but he didn’t.
A. could B. should C. ought to D. all the above
29. I missed the last bus, so I _____ go home on foot.
A. must B. have to C. may D. had to
30. He ought to win the first prize, _____ he?
A. oughtn’t B. shouldn’t C. mustn’t D. both A and B
参考答案：
1-5. D A C D D 6-10. B B B C B 11-15 C A B D D 16-20 C D D B A 21-25 D C B D B 26-30 C C D D B
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