Can' is most often used to ask for or give permission but 'may' and 'could' are also possible even though they are not used as often as 'can'.
Obligation:'Have to' and 'must' are both used to express obligation. There is a slight difference in the way that they are both used. 'Have to' shows that the obligation comes from someone else, not the speaker. This is usually referring to a rule or law. We have to be at the airport at least two hours before the flight.
I have to work on Saturday.
They have to wear their uniforms at school.
'Must' shows us that the obligation comes from the speaker.
I must hand in my thesis by tomorrow.
I really must call my parents.
INTERROGATIVE
HAVE TO - HAS TO
NEGATIVE
HAVE TO - HAS TO
You don't have to smooke in doors
Subtopic
Prohibition:'Can't' and 'mustn't' (must not) are used to show that something is prohibited (not allowed) You can't go into that restaurant without a tie. You can't drive in this country unless you are over eighteen.
You mustn't use your phone in class.
'Can't' usually gives the idea of something that is against the rules. Mustn’t usually means that it is the speaker who is setting the rule.
