Modal Verbs: Have to - Must

Have to

(Also have got to) used to show that you must do sth: Sorry, I've got to go. Did she have to pay a fine? You don't have to knock - just walk in. I haven't got to leave till seven. First, you have to think logically about your fears. I have to admit, the idea of marriage scares me. Do you have to go? (Especially BrE) Have you got to go?
(Also "have got to" especially in BrE) used to give advice or recommend sth: You simply have to get a new job. You've got to try this recipe - it's delicious.
(Also "have got to" especially in BrE) used to say that sth must be true or must happen: There has to be a reason for his strange behaviour. This war has got to end soon.
Used to suggest that an annoying event happens in order to annoy you, or that sb does sth in order to annoy you: Of course, it had to start raining as soon as we got to the beach. Do you have to hum so loudly? (= It is annoying).

Must

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). (Formal) 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. (Especially BrE) 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."
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).
(Especially BrE) used to recommend that sb do sth because you think it is a good idea: You simply must read this book. We must get together soon for lunch.
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 NAmE, 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 must finish this essay today. I'm going out tomorrow. I have to finish this essay today. We have to hand them in tomorrow.
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.
Both must and have to: are used to say that you are certain about something. "Have to" is the usual verb used in NAmE 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!