To express permission, prohibition, obligation and no obligation we usually use modal verbs.
當(dāng)表達(dá)允許、禁止、義務(wù)或者無(wú)義務(wù)的時(shí)候，我們通常用情態(tài)動(dòng)詞。

Permission
允許

Can is often used to ask for and give permission.
Can 經(jīng)常用于請(qǐng)求是否允許。

  • Can I sit here?
  • You can use my car if you like.
  • Can I make a suggestion?

We can also use may and could to ask for and give permission but can is used more often.
我們也用may和could去詢問(wèn)是否允許，但是can更常用。

Prohibition
禁止

Both can’t and mustn’t are used to show that something is prohibited – it is not allowed.
Can’t和mustn’t 是用于形容某事是禁止的—是不被允許的。

  • ?You can’t park here, sir.
  • You can wear jeans but you can’t wear trainers in that bar.
  • You mustn’t speak when the teacher is speaking.

Can’t tells us that something is against the rules. Mustn’t is usually used when the obligation comes from the person who is speaking.
Can’t表示某事是不符合規(guī)定的。Mustn’t經(jīng)常表示說(shuō)話人規(guī)定不可以做該事，即該義務(wù)來(lái)自于說(shuō)話人。

Obligation
義務(wù)

Have to and must are both used to express obligation. There is a slight difference between the way they are used.
“have to”和“must”都用于表達(dá)義務(wù)，但是兩者在用的時(shí)候有些許的不同。
Have to shows us that the obligation comes from somebody else. It’s a law or a rule and the speaker can’t change it.
“have to”表示該義務(wù)來(lái)自于他人，可能是法律或者規(guī)定，說(shuō)話人不可能改變的義務(wù)。

  • Do you have to wear a uniform at your school?
  • John can’t come because he has to work tomorrow.
  • In Britain you have to buy a TV licence every year.

Must shows us that the obligation comes from the speaker. It isn’t a law or a rule.
Must表示該義務(wù)來(lái)自于說(shuō)話人，而不是來(lái)自于法律或者規(guī)定。

  • ?I must call my dad tonight.
  • You must hand in your homework on Tuesday or your mark will be zero.
  • You must come and visit us the next time you come to London.

No obligation
無(wú)義務(wù)

We use don’t have to to show that there is no obligation. You can do something if you want to but it’s not compulsory.
我們用 “don’t have to”表明無(wú)義務(wù)，只要你想的話，你可以做某事，但是并不是義務(wù)范圍內(nèi)的意思。

  • You don’t have to wear a tie in our office. You can wear a tie if you want to but it’s OK if you don’t.
  • It’ll be nice if you do but you don’t have to come with me if you don’t want to.
  • You don’t have to dress up for the party. Wear whatever you feel comfortable in.