warn has 3 distinct synonyms spread across 1 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
advise or counsel in terms of someone's behavior
“I warned him not to go too far”
ask to go away
“The old man warned the children off his property”
notify, usually in advance
“I warned you that I would ask some difficult questions”
Synonyms, antonyms and the broader and narrower relations for warn come from Princeton University’s WordNet. Full sources and licences.