Hello ilayda, Those two words are synonyms and have the same exact meaning. However 'must' sounds more formal and sophisticated while have to tends to be more casual and can even sound child like if used Ex: You must help me, you are the only one You have to help me, I have no one else

'Have to' is used to express someone else's obligations. e.g. You have to pick her up at the airport in two days. 'Must' is used to express your own obligations. e.g. I must remember to hand in the paper on Monday.

"Must" is a modal. It has no past tense. It applies to present and future times by supplying a time reference. - I must leave now. - I must leave in 30 minutes. "To have to" is equivalent to must and has all the tenses. - I have to leave now. - I had to leave at noon. - I will have to leave at noon.