fix
quick
(colloquial) Quick; in a short time.
Ich muss nur noch fix die E-Mails checken, dann können wir gehen.
I just need to quickly check my emails, then we can go.

quick / fixed
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quick
(colloquial) Quick; in a short time.
Ich muss nur noch fix die E-Mails checken, dann können wir gehen.
I just need to quickly check my emails, then we can go.

The German adjective 'fix' means 'quick' or 'fixed'. It is a false friend of the English verb 'to fix', which in German means 'reparieren' (to repair).
Kannst du bitte mein Fahrrad reparieren?
Could you please repair my bicycle?
In the meaning 'schnell' (quick), 'fix' is mostly used colloquially and as an adverb. It then remains unchanged in the sentence and describes an action.
Warte kurz, ich hole nur fix meine Jacke.
Wait a moment, I'm just quickly getting my jacket.
In the meaning 'feststehend' (fixed), 'fix' is often used in economic or organizational contexts. Typical combinations are 'fixe Kosten' (fixed costs), 'ein fixer Termin' (a fixed appointment), or 'ein fixer Preis' (a fixed price).
Wir haben einen fixen Termin für das nächste Treffen vereinbart.
We agreed on a fixed date for the next meeting.
The expression 'fix und fertig sein' means to be completely exhausted. The two words 'fix' and 'fertig' are always used together here.
Nach dem Umzug war ich tagelang fix und fertig.
After the move, I was completely wiped out for days.
fast / quick
'schnell' and 'fix' can both mean that something happens in a short time or is done without a long pause. 'schnell' is neutral and fits almost everywhere: with movement, work, reactions, or processes. 'fix' is colloquial and sounds more casual. It is often used in everyday life when something is done quickly. In formal situations or written texts, 'schnell' is usually more natural.

Lisa schreibt fix eine Nachricht, bevor das Meeting beginnt.
Lisa quickly writes a message before the meeting begins.
Der Zug fährt heute besonders schnell durch die Landschaft.
The train is traveling especially fast through the countryside today.