avertir

To warn or notify someone, often about a danger, consequence, or upcoming change — je t'avertis que... A close neighbor of prévenir, with avertir leaning slightly more formal and often implying a warning with a consequence attached.

• Je vous avertis, ce chemin est dangereux la nuit. — I'm warning you, this path is dangerous at night.
• L'école a averti les parents du changement d'horaire. — The school notified parents of the schedule change.
• Il a été averti plusieurs fois avant d'être renvoyé. — He was warned several times before being fired.

Avertir and prévenir overlap heavily, but avertir carries a stronger sense of 'this is your warning, consequences will follow' — prévenir is more neutral and works better for simple advance notice, like prévenir qqn de son retard.

Often confused with prévenir.