Modal verbs
A modal verb is conjugated in the usual verb position. The other verb stays in the infinitive at the end: können (can), müssen (must), dürfen (may), wollen (want), sollen (should).
Understand the idea
Modal verbs add meanings such as ability, obligation, permission or intention. They are conjugated, while the other verb normally remains an infinitive. Several singular modal forms are irregular and do not follow the ordinary present-tense ending pattern.
Build it step by step
In a main clause, conjugate the modal in the normal finite-verb position and put the lexical infinitive at the end. The infinitive has no zu. Nicht dürfen expresses a prohibition; nicht müssen means that there is no obligation. Möchten is a common polite form used for wishes and requests.
See it in a sentence
Ich kann Deutsch sprechen.
Du darfst hier nicht rauchen.
A useful memory aid
Learn a modal together with a complete short sentence. Compare Ich kann kommen, Ich muss kommen and Ich darf kommen to feel the difference in meaning.
A common trap
Do not translate must not literally as nicht müssen. A prohibition usually needs nicht dürfen; lack of necessity uses nicht müssen.