Separable verbs can – as the name indicates – split into two parts. However, they also appear as one word in certain constructions.
When to separate
A separable verb splits when it is the only verb in a main clause. This applies to the following structures. Note that the prefix appears in the final position:
Er ruft seine Freundin um 8 Uhr an. He calls his girlfriend at 8 o’clock.
simple past tense
Ich kam am Dienstag zurück. I returned on Tuesday.
Steh sofort auf! Get/Stand up at once!
When not to separate
Separable verbs appear as one word and in the infinitive with modal verbs and the future tense:
Er muss seine Freundin anrufen. He has to call his girlfriend.
Ich werde am Dienstag zurückkommen. I’ll return on Tuesday.
In most subordinate clauses, the separable verb moves to the last position and does not split up. Its ending must agree with the subject of the clause:
Sag mir, wann du ankommst. Tell me when you’ll arrive.
Ich verstehe nicht, warum er sich immer so aufregt. I don’t understand why he always gets so upset.
The past participle of separable verbs is normally formed by inserting -ge- between the prefix and the main verb: ankommen → angekommen.