情态动词（can, have to, must, ought to, should）

北师大版高中英语必修1 Unit 3 lesson 3：

情态动词（can, , must, have to, ought to, should）

一、基本用法

(一) can的用法

1. 能力

I can speak English fluently.

注：can与be able to表能力的区别：

be able to表能力，不但有能力，而且实际做了。

Although he was badly hurt, he was able to escape from the fire.

2. 可能

(1) 理论上的可能性

Everyone can make mistakes.

(2) 一时的可能性; 有时会……

Scotland can be very cold sometimes.

3. 许可

(1) 陈述句：说话人和客观条件的许可。

You can park here.

You can’t smoke here.

(2) 疑问句

Can /May I help you? (请求许可)

Can he take the car out whenever he likes? (征求许可)

4. 推测（否定句/疑问句）

That can’t be Mary. ---She’s in hospital.

There’s someone outside. ---Who can it be ?

5. 过去式could; 否定式can’t.

(二) must和have to

must和have to都可表示义务、责任：

1. must是主观条件决定；have to是客观条件决定。

I must learn English regularly. (this is my own choice) （这是我自己的选择）

We have to get up very early to be at school on time.

(we have no choice because this is a rule) （我们没有选择，因为这是一个规则）

2. 可以用have to表示must不能表示的时态。

I had to go to bed early last night.

I hate having to get up early.

3. must的其它用法：

(1) 表示必须，应该

You must do everything as I do.

(2) 表示猜测（只用在肯定句中）

He must be an honest boy. 他一定是个诚实的男孩。

She must have studied English before.她以前一定学过英语。

(3) “偏要”“硬要”。表示说话人对句子主语所发出的动作或行为是不希望的、不满的甚至是生气的。通常指令人不愉快的事情。

Why must you be so stubborn (固执)？

If you must smoke, at least you could use an ashtray. (烟灰缸)

Why must you be so late ？你为何非要这么晚来呢 ？

(4) must 的否定式有三个：

★ 表示“不必”、 “用不着”时，用needn’t或don’t have to。

Must I go tomorrow?  No, you needn’t. 不，你不必去。

★ 表示“不可以”、 “禁止”时，用must not。

You mustn’t smoke in public places.你不可以在公众场所抽烟。

★ 表示推测时的否定式，用can’t。

He can’t be at home. I just saw him in the street.

(三)  ought to和should

1. 表示责任、义务 (= should)

Everyone ought to learn first aid.

2. 提建议 (= should)

You ought to buy a good dictionary.

3. ought to have done应该做某事，但没有做(= should)

These flowers died. You ought to have watered those flowers.

4. ought to 用于否定句、疑问句中

You oughtn’t to smoke so much.  

Ought you to smoke so much?

5. should的其它用法

(1) 某些从句中，表虚拟语气

I suggest that you should go there now.

(2) 用在why , how开头的疑问句中或that 引导的从句中, 表达说话人感情色彩，表达意外和不满，"竟，竟会"

Why should you be so late? 为什么你竟这么晚？

How should I know? 我怎么会知道？

I'm sorry that you should think so badly of me. 你竟把我想得那么坏，我感到遗憾。

(3) 推测, 意为“想必一定、照说应该、估计”等。

It’s 4:30. They should be in New York by now.  I should say she’s over forty.

二、同步练习

1. It is usually warm in my hometown in March, but it ______be rather cold sometimes.

A. must     B. can    C. should    D. would

2. You don't have to know the name of the author to find a book. You ______ find the book by the title.

A. must        B. need           C. can           D. would

3. She ______ have left school, for her bike is still here.

A. can’t         B. wouldn’t       C. shouldn’t    D. needn’t

4. My MP4 player isn't in my bag. Where ______ I have put it?

A. can     B. must   C. should   D. would

5. In crowded places like airports and railway stations, you ______ take care of your luggage.

A. can          B. may      C. must      D. will

6. John, look at the time. ______ you play the piano at such a late hour?

A．Must        B．Can         C．May     D．Need

7. --- It's the office! So you _____ know eating is not allowed here.

--- Oh, sorry.     

A. must      B. will    C. may   D. need

8. It_____ have been Tom that parked the car here, as he is the only one with a car.

A. may        B. can        C. must        D. should

9. --- May I smoke here?

--- If you ____, choose a seat in the smoking section.

A. should      B. could       C. may        D. must

10. According to the air traffic rules, you______switch off your mobile phone before boarding.

A. may     B. can            C. would       D. should

11. What do you mean, there are only ten tickets? There _____ be twelve.

A. should        B. would       C. will        D. shall

12. ---When can I come for the photos? I need them tomorrow afternoon.

---They _____be ready by 12:00.

A. can    B. should    C. might    D. need

13. This printer is of good quality. If it ______ break down within the first year, we would repair it at our expense.

A. would       B. should      C. could            D. might

14. --- My cat's really fat.

--- You ______ have given her so much food.

A. wouldn't      B. couldn't     C. shouldn't  D. mustn't

================  答案及解释 ===================

bcaac; aacdd; abbc