A very common confusion among German learners is to assume that the German verb müssen is used like the English verb must. This blog post clears up this confusion and tells you how müssen should be translated and used instead.
Müssen is one of six modal verbs in German. Modal verbs are used to express abilities, obligations, requests etc. They require an additional main verb at the end of a sentence in infinitive form. The modal verb müssen is best translated as "to need to" or "to have to" and should not to be confused with the English must for two reasons. One, the German müssen is not as strong as the English must. It is used to talk about necessities rather than legal obligations. Second, if müssen is used with the negation nicht it takes on a completely different meaning than the English must not.
Hunde müssen draußen bleiben.
(Dogs have to stay outside)
Ich muss meine Hausaufgaben machen.
(I need to do my homework)
Sie müssen den Müll trennen.
(They need to recycle)
Die Schüler müssen morgen nicht in die Schule gehen
(The pupils do not have to go to school tomorrow rather than the pupils must not go to school)
Ihr müsst nicht spazieren gehen, wenn ihr nicht wollt.
(You don't have to go for a walk, if you don't want to)
Musst du morgen nicht zur Arbeit gehen?
(Don't you have to go to work tomorrow)
While the first three examples are quite close to the English must, the last three show you that it is the much safer strategy to adopt "to need to" and "to have to" as translations for both positive and negative sentences.
Other common confusions among German learners are on the difference between müssen and brauchen in German and when infinitives need to be used with or without zu in German, so I devoted two blog posts to clearing up these confusions.
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