Obligation and permission | Johnny Grammar | Learn English | British Council

'Must' is used to express an obligation which the speaker agrees with:

You must study all this weekend for an exam on Monday.

'Have to' is used to express an obligation which has been imposed by someone else:

Question: Can you come out with me tomorrow?
Answer: I'm sorry. I have to study all weekend. I've got an important exam on Monday.

'Mustn't' means 'not allowed to':

You mustn't start writing till I say.

'Don't have to' and 'needn't' mean it is not necessary:

There are ten questions, but you don't have to answer them all.
You must answer at least five questions, but you needn't write more than 500 words for each answer.

'Must' doesn't have a past tense instead you use 'had to':

Question: Why are you looking so unhappy?
Answer: Because my boyfriend had to study all weekend.

Instead of 'mustn't' you use 'wasn't / weren't allowed to':

Quesion: Can I see your exam?
Answer: I'm sorry. I wasn't allowed to take it out of the exam room.

'Can' is used to ask for and give permission:

Question: Can I borrow your notes, please? I'll have the same exam next week.
Answer: Yes, of course you can, but you mustn't lose them!

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