Both words can be used to mean “necessary”, but according to some books, “have to” and “have got to” are much more common in American English than in British English. Though “have to” and “must” can be used to talk about an “obligation”, the source is different. When you tell someone, “We must wear a helmet”, it is you, the speaker, who is compelling the listener to wear one. The obligation comes from you. If on the other hand, you say, “We have to wear a helmet in Bangalore,” you are implying one is obliged to wear a helmet in that city because it is the law. In this case, the obligation comes, not from the speaker, but an outside source.
*I must finish my assignments. (I want to) *I have to finish my assignments. (My mother told me to finish them!) *Must you brush your teeth after every meal? (Is it personally important to you?) *Do you have to brush your teeth after every meal? (Is it a rule?)
Unlike “must”, “have to” is normally used to indicate that an individual does something regularly because it is a part of his job. “The young mother has to do all the housework when the baby is asleep.” In this context, “must” is not used. When used in the negative, the two words have very different meanings. When you tell an individual he must not smoke, you are ordering him not to do so. He has no choice regarding this matter; he is obliged not to smoke. The opposite of “have to” is “don’t have to.” When you tell someone that he doesn’t have to smoke, what you mean is that it is not necessary for him to smoke. There is no obligation for the individual to do so, but if he wishes to smoke, he can. He has a choice in this matter. In official documents, “must” is much more common than “have to.” “Candidates must send two photographs along with the application.” These are just some of the differences between the two words.