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情態(tài)動詞（情態(tài)助動詞）是協(xié)助主動詞構(gòu)成限定動詞詞組，無力例外地帶有不帶to的不定式。情態(tài)動詞多半是一詞多義；在肯定句、否定句和疑問句中，往往意義不盡相同。而且“時”不是時間區(qū)別的主要標(biāo)志，例如現(xiàn)在時的情態(tài)動詞不一定表示現(xiàn)在時間，過去時情態(tài)動詞也不一定表示過去時間。

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兩者區(qū)別

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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