müssen

See also: mussen and muessen

German

From (deprecated template usage) [etyl] Old High German muozan, from (deprecated template usage) [etyl] Proto-Germanic *mōtaną. The closeness to the original diphthong can for example still be seen in the Bavarian first and third person singular, where one says I muas (I must) or Ea/Si/Es muas (He/she/it must).

(auxiliary, with an infinitive → “müssen” replaces the past participle) to have to (do something); must; to be obliged (to do something); to need (to do something).
Wir müssen es machen. — “We must do it.”

Wir müssen es nicht machen. — “We need not do it.” or “We do not need to do it.”

Ich musste es machen. — “I had to do it.”
(intransitive, with past participle “gemusst”) to have to do something implied; must; to be obliged.
Ja, ich muss. — “Yes, I must.”

Ja, ich habe gemusst. — “Yes, I had to.”
(colloquial, euphemistic) to need to go to the bathroom.

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 acceptable spelling is with ss.