To express permission, prohibition, obligation and no obligation we usually use modal verbs.
当表达允许、禁止、义务或者无义务的时候，我们通常用情态动词。

Permission
允许

Can is often used to ask for and give permission.
Can 经常用于请求是否允许。

  • 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.
我们也用may和could去询问是否允许，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—是不被允许的。

  •  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表示某事是不符合规定的。Mustn’t经常表示说话人规定不可以做该事，即该义务来自于说话人。

Obligation
义务

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”都用于表达义务，但是两者在用的时候有些许的不同。
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”表示该义务来自于他人，可能是法律或者规定，说话人不可能改变的义务。

  • 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表示该义务来自于说话人，而不是来自于法律或者规定。

  •  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
无义务

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.
我们用 “don’t have to”表明无义务，只要你想的话，你可以做某事，但是并不是义务范围内的意思。

  • 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.