müssen meaning and definition in german

Müssen can express the necessity of doing something, but also the moral duty. Sollen expresses moral duty or the suggestion that something ought to be done. Both verbs can express a duty imposed by someone else. In this case müssen is stronger than sollen, implying that the imposing person has some kind of power to make the other really do it.
As shown in the first line, the infinitive is used instead of the past participle in the compound tenses if the verb has a complement verb. The phenomenon is called double infinitive.
Das hätten wir machen müssen.
We should have done that.
In southern German and Austrian speech, the infinitive is often used instead of the past participle in any construction. Thus: ich habe nicht müssen for standard ich habe nicht gemusst.
As shown in the example above, nicht müssen translates into English as “need not” rather than “must not”. For the English “must not”, use nicht dürfen.
Before 1996, several of the conjugated forms of müssen were spelled with an ß (that is, muß, mußte, müßte, etc.). Since the spelling reform of 1996, however, the correct spelling is with ss.