"Must" and "have to" have the same meaning, for example: "You must eat your cabbage" and "You have to eat your cabbage" mean the same - you are required to eat your cabbage. "Mustn't" and "don't have to" are not quite the same. "You mustn't eat your cabbage" means you are required not to eat it, you are forbidden to eat it. "You don't have to eat your cabbage" means you're not required to eat it, but you're not forbidden from eating it either - you can choose to eat it or not. There is a negative form of "have to" that would mean the same as "mustn't" - we could say "have not to", as in "You have not to eat your cabbage", but this construction feels unnatural to native speakers and so would not commonly be used.

Hello. Mustn't----Something is forbidden,we say mustn't Don't have to----means"need not to"