Modal verb müssen

Expressing lack of necessity or prohibition

The negation of müssen expresses a lack of necessity, particularly in reference to the meaning of the infinitive. In this case, nicht müssen can be replaced by nicht brauchen. The infinitive takes the word zu.

When müssen expresses a request/command (which, in a broader sense, is also a necessity) through negation (i.e. a prohibition or an order not to do something), it can be substituted by nicht sollen or nicht dürfen.

These two verb pairs, that express lack of necessity or prohibition, differ as far as their intensity is concerned.