In the affirmative have to / must both express the idea of obligation.(It is necessary that you do something.) However, must is only used in the present, and is never used after 'will' or 'may.'
-I must / have to get up early because I start work at 8 a.m.
-I will must / have to leave work early if the snowstorm continues.
In the negative only the verb have to (do/does not have to) expresses the idea of obligation:(It is not necessary that you do something.)
-You don't have to pay for children. They can come in for free.
In the negative must (must not / mustn't) is similar to an imperative: Do not do something:
-You mustn't smoke in the corridors. (= Don't smoke in the corridors.)
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