sure has 9 distinct synonyms spread across 7 of its 10 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 one recorded opposite is unsure.
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.
exercising or taking care great enough to bring assurance
“be certain to disconnect the iron when you are through”
definitely or positively (`sure' is sometimes used informally for `surely')
“the results are surely encouraging”
3 of the 10 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for sure.
Synonyms, antonyms and the broader and narrower relations for sure come from Princeton University’s WordNet. Full sources and licences.