past has 6 distinct synonyms spread across 4 of its 6 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. Its 2 recorded opposites are future, present.
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.
Synonyms, antonyms and the broader and narrower relations for past come from Princeton University’s WordNet. Full sources and licences.
a earlier period in someone's life (especially one that they have reason to keep secret)
“reporters dug into the candidate's past”
a verb tense that expresses actions or states in the past