"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.)