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Must or have to?

"Must" is used to talk about necessity and obligation, e.g. You must do your homework.
"Must" is also used to express the conclusion that something is certain, e.g. You must be very tired after the long journey.
In British English "must' is also used to give strong advice, e.g. You really must stop smoking. Sometimes it sounds like an invitation, e.g. You must come and visit us.

"Have to" is used to express external obligation, e.g. I have to finish my homework.
"Have to" is also used in a situation when you don't have any other choice, e.g. I've just missed the last bus, so now I have to walk three miles.

Study the following example sentences:

I must see the film. I've heard it's great. (This is what I really want to do.)
I have to get up at 5 o'clock five days a week. (I have no choice, it's my obligation.)
I must stop eating sweets. (It's my decision, this is what I want.)
I have to stop eating sweets. (My doctor told me to do it.)
You must write the essay for tomorrow. (said by the teacher)
We have to write the essay for tomorrow. (said by the students)
She must be very rich. (It's my conclusion. I'm sure she's rich.)
The policemen have to check every passing car. (That is an official order.)

Remember that it is possible to give orders with "must" or "have to", so if you are the boss (a parent, a teacher, etc.) you can say: "You must do it" or "You have to do it". When giving orders "must" seems to be more formal than "have to". When you are obliged to do something, you can only use "have to".