
prompt
Happening immediately, right away, and without delay. Often used as an adverb.
Die Lieferung erfolgte prompt am nächsten Tag.
The delivery arrived promptly the next day.
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Happening immediately, right away, and without delay. Often used as an adverb.
Die Lieferung erfolgte prompt am nächsten Tag.
The delivery arrived promptly the next day.
'Prompt' (prompt/promptly) is almost always used as an adverb to describe that an action happens immediately and without delay. It answers the question 'How?'. Use as an adjective before a noun, e.g. 'eine prompte Antwort' (a prompt answer), is rare.
Auf meine E-Mail hat er prompt geantwortet.
He replied promptly to my email.
Although it resembles the English word 'prompt', the German word 'prompt' is only an adjective or adverb. It cannot be used as a verb, as in English 'to prompt someone' ('jemanden zu etwas veranlassen').
Ihre Frage veranlasste ihn zu einer schnellen Antwort.
Her question prompted him to give a quick answer.
Common verbs combined with 'prompt' (promptly) are 'antworten' (to answer), 'reagieren' (to react), 'liefern' (to deliver), and 'bezahlen' (to pay). It describes a quick and expected response.
Der Kundenservice reagierte prompt auf meine Beschwerde.
Customer service responded promptly to my complaint.
The word 'prompt' (prompt/promptly) is often used in more formal or business contexts. It sounds somewhat more refined than the everyday word 'sofort' (immediately).
Wir erwarten eine prompte Erledigung des Auftrags.
We expect prompt completion of the order.
fast / quick
Schnell can refer both to a high speed and to a short reaction time. Prompt emphasizes that a response happens immediately and without delay. When the main focus is the short time needed to carry something out, schnell is suitable; for reacting immediately, prompt is more precise.

Direkt nach Eingang der Anfrage antwortete sie prompt.
She replied promptly as soon as the inquiry came in.
Sobald sie mit dem Schreiben begann, verfasste sie die Antwort schnell.
As soon as she began writing, she composed the response quickly.