have to = need to I have to eat now= I need to eat now. Must= importantly need to I must eat now= I (importantly) need to eat now. hope this helps. :)

Use "have to" when the obligation comes from somewhere else: another person, or the situation. Use "must" when the obligation comes from within: for your own sake. This is the basic difference as described in grammar textbooks. If you're facing an exam, this is the exact rule to follow. PS. "have to" and "need to" are two different things: "have to" is an obligation; "need to" is a necessity.

in some cases, they can be interchangeabl. example, 1 "you must go to bed now" 2 "you have to go to bed now" (more common) but in other uses they are different, "surely you must want something for your birthday" if you replace "must" with "have to" in this sentence it does not make sense