warn vs forewarn

warn and forewarn both are verbs.

Word	Noun	Adjective	Verb	Adverb
warn	No	No	Yes	No
forewarn	No	No	Yes	No

As verbs, forewarn is a hyponym of warn; that is, forewarn is a word with a more specific, narrower meaning than warn:

Other hyponyms of warn include previse, admonish, caution, monish, threaten, alarm, alert.

warn (verb)	forewarn (verb)
notify of danger, potential harm, or risk	warn in advance or beforehand; give an early warning
advise or counsel in terms of someone's behavior
notify, usually in advance
ask to go away