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知识体系>>语法部分

(一)

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.

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.

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