abgehen
to depart
To leave a place; to move away from a place. Often used for means of transport or people.
Der Bus zum Flughafen geht alle 30 Minuten ab.
The bus to the airport departs every 30 minutes.

to depart / come off
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to depart
To leave a place; to move away from a place. Often used for means of transport or people.
Der Bus zum Flughafen geht alle 30 Minuten ab.
The bus to the airport departs every 30 minutes.

'Abgehen' (to depart / come off) is a separable verb. In a main clause, the prefix 'ab' goes at the end of the sentence. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Der letzte Zug nach Berlin geht um 22 Uhr ab.
The last train to Berlin departs at 10 p.m.
The perfect tense of 'abgehen' (to depart / come off) is always formed with the auxiliary verb 'sein' because it describes movement or a change of state.
Der Bus ist gerade pünktlich abgegangen.
The bus has just departed on time.
The verb consists of the prefix 'ab-' (away) and 'gehen' (to go). The basic idea that something moves away from or detaches from a point appears in many of its meanings.
Wann geht der nächste Flug nach London ab?
When does the next flight to London depart?
When something unintentionally comes off, the person this happens to is often named in the dative. The thing that comes off is the subject of the sentence.
Beim Essen ist mir ein Stück vom Zahn abgegangen.
While eating, a piece of my tooth came off.
In the meaning 'abweichen' (to deviate), 'abgehen' (to deviate) is always used with the preposition 'von' and the dative.
Wir dürfen nicht vom Thema abgehen.
We must not deviate from the topic.
In an educational context, 'abgehen' (to leave) means to leave a school, often before the regular graduation. Here too, the preposition 'von' is used.
Sie ist nach der 11. Klasse von der Schule abgegangen, um eine Ausbildung zu machen.
She left school after 11th grade to do vocational training.
The meaning 'großartig sein' (to be great) is purely colloquial. It is used for parties, concerts, or other high-energy events.
Das Konzert gestern war der Hammer, die Band ist total abgegangen!
The concert yesterday was awesome; the band totally went off!
'Was geht ab?' (what's up?) is a fixed, very informal greeting among friends, similar to 'Was ist los?' (what's going on?) or 'Wie geht's?' (how are you?).
Hey, lange nicht gesehen! Was geht ab?
Hey, long time no see! What's up?
to leave / rely
'verlassen' and 'abgehen' can both have to do with leaving a place. 'verlassen' is the normal, general verb for people, places, and groups. 'abgehen' is more specific: it is often used for means of transport, official processes, or leaving school. Learners may confuse the words because both can sometimes mean 'to go away'. But in many everyday sentences, 'abgehen' does not sound natural. For a person leaving a room or a party, you usually say 'verlassen'. With a school or means of transport, 'abgehen' can fit.

In der Schule gehen jedes Jahr mehrere Schüler nach der zehnten Klasse ab.
At the school, several students leave every year after tenth grade.
Jan verlässt die Feier früh, weil er am nächsten Morgen reisen muss.
Jan leaves the party early because he has to travel the next morning.