情态动词的用法

 情态动词(Modal Verbs)用来表示能力、允许、许诺、可能、必须、劝告、意愿等概念和态度。主要的情态动词有can (could), may (might), shall (should), will (would), must, ought to, need, dare等。情态动词没有人称和数的变化，其后常跟不带to的不定式。

一、表示能力

1. can/ could “能够”、“会”

e.g. I) Two eyes can see more than one.

II) She couldn’t attend the English class yesterday.她昨天不能去上英语课。

III) She can’t attend the English class tomorrow.

2. can/could与be able to比较

a) be able to可以用现在完成时、不定式、动词-ing形式等，can/could则不可以。

e.g. I) The teacher is very busy; he isn’t able to help me now (or he can’t help me).

II) He was able to follow the teacher. (or he could follow the teacher.)

III) He will be able to swim some day. (or he can swim some day.)

IV) This is the only ticket I’ve been able to get so far.

V) I used to be able to play cards well.

b) could表示过去总的、经常性的能力；was/were able to表示过去具体情况下具有的能力，并且做了某动作，这种情况下，不能用could。

e.g. I) He could speak Italian, so he was able to bargain (讨价还价) with the storekeeper.

II) How many eggs were you able to get? (were you able to不能改为 could you get )

 

二、表示许可

1. 征询“许可”或给予“许可”可用can/could或may/might表示。may用于正式场合，can用于非正式场合，could用于客气的询问，might则很少用。

e.g. I) You can surf the net (上网) for an hour.

II) Could I borrow your dictionary?

III) Might I trouble you for a light? 我能借个火吗？

2. 表示“不允许”用can’t, may not, mustn’t, 其中mustn’t语气重，表示“不许”。

e.g. I) You can’t smoke here.

II) You mustn’t smoke near a gas station (加油站).

3. 表示过去某次特定的“许可”不用could, might, 要用其它表达方式。

e.g. I) was allowed to see her yesterday evening. 昨晚我被允许去看她。(was allowed to see不能改为 could see)

II) He had permission to go out for an hour. 他得到许可出去一小时。(had permission to go不能改为could go out)

三、表示可能性

1. may, might用来推测现在“可能”；may必might表示的可能性更大。

e.g.– Why is Jack absent from class?

-- He may be ill. (他可能生病了。)/ He might be sick. (他也许生病了。)

注：maybe是副词，不是情态动词，注意不要混淆了。

e.g. Maybe he is sick.

2. 在口语中常用can/could表示“可能”。

e.g. I) Your anger can kill you.

II) According to the radio it could (may, might) rain this evening. 收音机里说今晚可能要下雨。

III) It can’t be true. She must be mistaken. 这不可能是真的，她一定是弄错了。（在这个意义上，can't 是must的反义词。）

3. must 表示的可能性最大，表示“一定”，“肯定”；否定形式用can’t, 表示“一定不”，“肯定不”。

e.g. I) You must be very thirsty (口渴的) after much exercise.

II) She can’t be home, for no one answer the phone.

四、表示必须和需要

1. must

a) must表示说话人认为“必须”；否定回答时用needn’t或don’t have to，表示“不必”。

e.g. I) – Must you go so soon?

-- Yes, you must./ No, you needn’t.

II) We must do everything step by step.

b) mustn’t表示“不许”，“一定不要”。

e.g. I) You mustn’t talk like that.

II) We mustn’t stay here any longer.

2. have to

a) 表示他人或客观情况要求必须。

e.g. I) You don’t have to pay taxes. 你不必缴税的。

II) I have to be back at home by seven. I have a lot of work to do tonight.

b) have to除有现在时外，还有一下不同时态形式。

e.g. I) He had to leave.

II) He will have to go for a job interview (工作面试) this afternoon.

III) As his father was a poor man, he would have to work after school.

c) 口语中常用have got to表示“必须”。

e.g. I have got to hurry up; otherwise I won’t catch the last bus.

3. need

a) 情态助动词need表示需要，只用作本体意义上的must的否定和疑问形式，没有人称和数的变化，后接省略to的不定式，否定式为needn’t。

e.g. I) You needn’t try to explain.

II) -- Need I go now?

-- Yes, you must./ No, you needn’t.

b) 行为动词need作实意动词时，有人称和数的变化，后接带to的不定式，否定式要在前面加don’t（doesn’t，didn’t）。疑问句用do（does，did）提问。

e.g. I) People need to get all the essential nutrients. 人需要基本的营养物质。

II) Does she need to go?

III) You didn’t need to tell him everything you knew.

五、表示责任和劝告

1. should和ought to都可以用来表示责任和劝告，后者的语气重些。ought to否定式为ought not to 或oughtn’t to, 疑问式把ought放在主语前。

e.g. I) She ought to/should understand.

II) Ought we to go now? (or should we go now?) 我们必须走吗？

III) It oughtn’t to/ shouldn’t take much longer. 这该不会花很长时间。

2. must有时也可以用来表示责任和劝告。

e.g. You must go to school today.

3. 口语中也常用had better表示劝告，意为“最好做/不做某事”，后接动词原形，其否定是为：had better not do sth.，反意疑问句则用：hadn’t …。

e.g. I) You’d better spend at least 2 hours learning English.

II) You’d better not drink cold water.

III) Hadn’t you better follow the doctor’s advice?

六、表示建议

1. “Shall I/ we … ?”表示征求对方对建议的看法。

e.g. I) Shall I close the window?

II) Shall we bring anything? Some wine?

III) Let’s go, shall we?

2. may/might (just) as well表示推荐最佳方案，意为“倒不如……”、“……不也一样吗？”。may/might well的意思为“很有可能……”，注意不要混淆。两者都接不带to的不定式。

e.g. I) All the pubs are closing -- we may as well go home. 所有的酒吧都关门了，我们还不如回家呢。

II) These qualities may well make a great athlete. 这些品质很有可能会造就一名伟大的运动员。

七、表示意愿

1. you/ he/ they shall表示说话人的强烈意愿，有时甚至有威胁的含义。说话人“一定要”他人做某事。

e.g. I) You shall pay for this. 你一定要为此付出代价！

II) They shall do what I tell them to do.

2. will/ would表示动作者“愿意”。

e.g. I) I’ll do it, if you like. 如果你喜欢，我愿意做这件事。

II) If they will do this, they will probably add years to their lives. 如果他们愿意这么做，就可能延长寿命。(第一个will表示意愿，第二个表示将来。)

3. “will you/ would you …?”表示客气的建议、询问。

e.g. I) Would you mind if I close the window?

II) Would you like a cup of coffee?

4. would还可以用来表示过去的习惯。

e.g. Sometimes he would walk slowly by the dining-room doors.

八、表示勇敢

1. 情态动词dare在现代英语中并不常用，作为情态动词，意思是“敢于”，没有人称和数的变化，后接省略to的不定式，否定式为dare not（或daren’t）。

e.g. I) She daren’t tell the boss because she doesn’t want to make trouble.

II) How dare you talk to your teacher that way?

2. dare作为实意动词，意思是“敢于”，“向……挑战”，形式上有人称和数的变化，理论上后接带to的不定式，在实际运用中to也常常省略。否定式要在前面加don’t/ doesn’t/ didn’t。疑问句用do/does/did提问。

e.g. I) Sally is the only person in our class who dares (to) answer Miss Thompson back. 萨利是我们班里唯一敢跟汤姆森老师顶嘴的人。

II) I have never dared to tell him about it.

九、情态动词后的某些动词形式

1. 情态动词有时可以和动词的完成形式构成谓语。

a) 用于表示推测过去某动作的可能性，可以用“can, could, may, might, must + have done”表示，其中might的可能性最小，must的可能性最大。must只能用在肯定猜测中，而can’t/couldn’t可以表示“否定”的猜测，表示“一定没有发生”。

e.g. I) He may/might have missed the train.

II) They couldn’t have gone so far。他们不可能做到这样的地步。

III) The dictionary was not there. Who could have borrowed it?

IV) You screamed (尖叫) in your sleep last night. You must have had a nightmare (恶梦) .

b) “should/ ought to + have done”表示“不该做而实际并未做的动作”，常含有批评的意思。“shouldn’t/ oughtn’t to + have done”表示“本不该做而实际却已经做的动作”，也含有批评的意思。

e.g. I) You should have stopped her watching the film. 你本来应该阻止她看这部电影的。

II) They ought to have stopped at the traffic lights. 他们本来应该在红绿灯灯前停车的。

III) You shouldn’t have told anyone about this matter.

c) “needn’t + have done”表示一种已经做过但没有必要的行为。

e.g. I) You needn’t have watered the flowers, for it is going to rain. 你本来没必要浇花，因为马上要下雨了。

II) – We went to the meeting, but it had been cancelled (取消).

-- Well, so you needn’t have gone. (本来用不着去的。即：你去了，但是本不必去的。)

注意区别didn’t need to。

e.g. – We were getting ready to go to the meeting, but he telephoned me that it was cancelled.

-- Well, so you didn’t need to go. (你不必去了。即：你没有去，也没有必要去。)

d) “would + have done”主要       用于虚拟语气中，详见虚拟语气。

2. 情态动词后可以接动词的进行式，表示对现在进行的事物的猜测。

e.g. I) He may/ be waiting for you.

II) He can’t be playing tennis in such weather.

III) He must be studying in the library now.

3. 情态动词后接不定式完成进行时，表示对过去某动作是否一直在进行的推测或评论。

e.g. I) She must have been working till small hours in the morning (凌晨).

II) She couldn’t have been swimming all day.

III) They may have been discussing the problem this afternoon.