must modal at English => English (Oxford Advanced Learners) Of Explained:
modal verb )
1 used to say that sth is necessary or very important (sometimes involving a rule or a law):
All visitors must report to reception. * Cars must not park in front of the entrance (= it is not allowed). * I must ask you not to do that again. * You mustn't say things like that. * I must go to the bank and get some money. * I must admit (= I feel that I should admit) I was surprised it cost so little. * Must you always question everything I say? (= it is annoying) * 'Do we have to finish this today?' 'Yes, you must.'
HELP NOTE Note that the negative for the last example is 'No, you don't have to.
2 used to say that sth is likely or logical:
You must be hungry after all that walking. * He must have known (= surely he knew) what she wanted. * I'm sorry she's not here. She must have left already (= that must be the explanation).
3 used to recommend that sb does sth because you think it is a good idea:
You simply must read this book. * We must get together soon for lunch. -> MODAL
if you must (do sth) used to say that sb may do sth but you do not really want them to:
'Can I smoke?' 'If you must.' * It's from my boyfriend, if you must know.
noun [usually sing.] (informal) something that you must do, see, buy, etc:
His new novel is a must for all lovers of crime fiction. * Trips to Pompeii and Naples are absolute musts.
must / have (got) to / must not / don't have to
Necessity and Obligation
Must and have (got) to are used in the present to say that something is necessary or should be done. Have to is more common in AmE, especially in speech: You must be home by 11 o'clock. * I must wash the car tomorrow. * I have to collect the children from school at 3 o'clock. * Nurses have to wear a uniform.
In BrE there is a difference between them. Must is used to talk about what the speaker or listener wants, and have (got) to about rules, laws and other people's wishes: I'd love to stay, but I must go now - I've got to write an assignment for my tutor this evening.
There are no past or future forms of must. To talk about the past you use had to and has had to: I had to wait half an hour for a bus. Will have to is used to talk about the future, or have to if an arrangement has already been made: We'll have to borrow the money we need. * I have to go to the dentist tomorrow.
Questions with have to are formed using do: Do the children have to wear a uniform? In negative sentences both must not and don't have to are used, but with different meanings. Must not is used to tell somebody not to do something: Passengers must not smoke until the signs have been switched off. The short form mustn't is used especially in BrE: You mustn't leave the gate open. Don't have to is used when it is not necessary to do something: You don't have to pay for the tickets in advance. * She doesn't have to work at weekends.
-> note at NEED
Both must and have to are used to say that you are certain about something. Have to is the usual verb used in AmE and this is becoming more frequent in BrE in this meaning: He has (got) to be the worst actor on TV! * This must be the most boring party I've ever been to (BrE). If you are talking about the past, use must have: Your trip must have been fun!