Usage of "müssen"
The verb "müssen" corresponds to English "must" or "to have to". For example:
Ich muss abnehmen.
(I must lose weight.)
Am Abend muss ich immer lernen.
(In the evenings I have to study.)
The negative of "dürfen" and "müssen"
Great care must be taken when using "dürfen" and "müssen" in the negative. Although "müssen" is used to translate "must", it is "nicht dürfen" which is used to translate "mustn't" in the sense of "not being allowed to do something".
The words "nicht müssen" are instead used to translate "don't have to". For example:
Michael darf das nicht tun.
(Michael must not do that.)
Michael muss das nicht tun.
(Michael does not have to do that.)
Alkohol darf ich nicht trinken.
(I'm not allowed to drink alcohol.)
Alkohol muss ich nicht trinken.
(I don't have to drink alcohol.)
