verb
to warn, to caution
B1
Warnen: 'to warn' or 'to caution' someone about danger or consequences. Regular weak verb; auxiliary haben (Partizip: gewarnt). Not reflexive or separable. Often used with the preposition 'vor' (jemanden vor etwas warnen), common in safety notices, advice or cautions in speech and writing.
Examples
Das Schild warnt vor gefährlichen Steinstürzen.
The sign cautions against dangerous rockfalls.
Der Polizist warnte die Passanten, damit niemand die Absperrung überquerte.
The policeman warned the passersby so that no one crossed the barrier.
Ich warne dich vor der Gefahr.
I'm warning you about the danger.
Details
Mnemonics
picture a triangle warning sign with the word 'warnen' flashing
sounds like English 'warn' — same root
n/a
Notes
warnen is a regular (weak) verb. In many contexts 'warnen vor' is used with dative for the thing being warned about (e.g., jemanden vor etwas warnen).