Synonyms for Warn:

n.

whistle, lecture, sound/raise the alarm, portend, warn off. forewarn (noun)
give the alarm, predict, tip off, study at adviseadvise, offer a word of caution, remind, put on guard, forearm, urge, make red lights flash, signal, forbid, give the high sign, cry wolf, give notice, put on the alert, prepare, put wise, prepare for the worst, remonstrate, sound the alarm, apprise, start bells ringing, read the riot act, cry havoc.

v.

advise (verb)
caution, consult, advise, brief, notify, confer, guide, admonish, suggest, inform, instruct, prompt, cue, recommend, counsel, advocate, direct, encourage.
caution (verb)
enjoin, exhort, Forewarn, alert.
give notice of possible occurrence (verb)
instruct, exhort, prompt, admonish, suggest, cry wolf, inform, tip off, forbid, hint, caution, apprise, prepare, guide, advise, remonstrate, recommend, signal, put on guard, give the high sign, counsel, alert, urge, threaten, direct, remind, forearm, advocate, Forewarn, predict, notify, enjoin.
threaten (verb)
frighten, hint, harass, forebode, scare, intimidate, torment, browbeat, bulldoze, bully, threaten.
warn (verb)
alarm.

Other synonyms:

prepare, warn off, give notice. tip off, remind. whistle, lecture. advise
tip off, prompt.
dissuade
remonstrate.
endanger
threaten.
forecast
portend.
remind
remind.
Other relevant words:
lecture, predict, cry wolf, forbid, forearm, give the alarm, whistle, put on guard, signal, remind, cry havoc, apprise, urge, give the high sign, remonstrate, prepare, give notice, sound the alarm, portend.

Usage examples for warn

  1. What did Honor warn you about? – Banked Fires by E. W. (Ethel Winifred) Savi
  2. He carried them on his heart, and he says, " That by the space of three years I ceased not to warn every one night and day with tears." – Godliness by Catherine Booth