"Have to" and "must" have the same meaning in the affirmative and interrogative forms when referring to obligation. Some people think that "must" is slightly stronger, but for all practical purposes, they mean the same thing: Examples: 1.Doctors have to attend medical school for several years before they can practice medicine. 2.Doctors must attend medical school for several years before they can practice medicine. Both verbs can be used in British English to talk about obligation. (In American English, 'have to' is the normal form.) British English often makes a distinction as follows. 'Must' is used mostly to talk about the feelings and wishes of the speaker and hearer — for example, to give or ask for orders. 'Have (got) to' is used mostly to talk about obligations that come from "outside" — for example from laws, regulations, agreements and other people's orders. Examples: 1.I must stop smoking. (I want to.) 2.I have to stop smoking. (Doctor's orders.)

There's only a slight difference between must and have to, and so the two can be used interchangeably. If you really want a detailed understanding of the two, then it is commonly said that 'have to' often expresses an obligation that comes from the outside, while 'must' expresses an obligation that comes from the speaker's feelings and wishes. For example: I must go on a diet, because I feel that I should. I have to go on a diet, because the doctor said so. Must you wear all black all the time? (implies that it is the personal choice of the person wearing all black) Do you have to wear all black all the time? (implies that there is a rule that the person must wear all black)