北京PET语法点10期：情态动词Modal verbs:should,shouldn't,must,mustn't,have to等

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情态动词（情态助动词）是协助主动词构成限定动词词组，无力例外地带有不带to的不定式。情态动词多半是一词多义；在肯定句、否定句和疑问句中，往往意义不尽相同。而且“时”不是时间区别的主要标志，例如现在时的情态动词不一定表示现在时间，过去时情态动词也不一定表示过去时间。

⑴ should/shouldn't

We use should/shouldn't to give or ask for advice.Should/shouldn't are followed by the infinitive without to.

例句：

You should join a gym if you want to keep fit.

You shouldn't eat too much chocolate.

What should I do ?

注意：

An alternative to should is ought to +infinitive.This is more formal.

⑵ must, have to 

We use must and have to to express obligation.

例句：

You must take your passport with you when you travel abroad.

We have to show our passports when we cross  the border.

注意：must和have to两者区别

We often use must to talk about rules or laws which we agree with or believe in.

例句：

We must wear a seat belt in the car, even for short journeys.

We often use have to to talk about rules or laws which were made by someone else or which we may not agree with.

例句：

My teacher says that I have to finish the homework tonight or I'll be in trouble.

⑶ don't have to 

Use don't have to or needn't(but not mustn't)when it's not necessary to do something.

例句：

We didn't have to show our passports when we went to Scotland.

We don't need to wear a uniform at our school.

⑷ can/can't

We can also use can/can't to express permission or lack of permssion.

例句：

You can leave any time you like.

You can't bring animals in here.

Can I use your phone,please?

⑸musn't

Use mustn't to express prohibition,to say that something is not allowed.

例句：

You mustn't use your phone in the cinema.

注意：

We can also use can't instead of mustn't.

例如：You can't talk during the exam.

There is no past tense form of mustn't.Use not allowed to.

例如：We weren't allowed to go into the concert without a ticket.

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