To express permission, prohibition, obligation and no obligation we usually use modal verbs.
Can is often used to ask for and give permission.
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.
Both can’t and mustn’t are used to show that something is prohibited – it is not allowed.
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.
Have to and must are both used to express obligation. There is a slight difference between the way they are used.
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.
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.
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.
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.