Separable verbs
In a main clause the stressed prefix of a separable verb goes to the end. With a modal, the complete infinitive stays together at the end.
Understand the idea
A separable verb has a prefix that can leave the finite verb in a main clause. The prefix still belongs to the verb's meaning. Stress and the dictionary entry help distinguish separable verbs from inseparable ones; spelling alone is not always enough.
Build it step by step
In a main clause, conjugate the verb stem and put the separated prefix at the end. With a modal, the infinitive stays together. In a verb-final subordinate clause, the finite separable verb is written as one word. In a zu infinitive, zu usually goes between a separable prefix and the stem.
See it in a sentence
Ich kaufe im Supermarkt ein.
Ich muss früh aufstehen.
A useful memory aid
Learn the infinitive, a short main clause and the participle as a family: aufstehen, ich stehe auf, aufgestanden.
A common trap
Do not separate the infinitive after a modal. Also avoid treating every initial syllable as separable: besuchen behaves differently from anrufen.