simple has 13 distinct synonyms spread across 6 of its 9 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 complex, compound.
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.
having few parts; not complex or complicated or involved
“a simple problem”
easy and not involved or complicated
“an elementary problem in statistics”
exhibiting childlike simplicity and credulity
“childlike trust”
lacking mental capacity and subtlety
(botany) of leaf shapes; of leaves having no divisions or subdivisions
any herbaceous plant having medicinal properties
1 of the 9 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for simple.
Synonyms, antonyms and the broader and narrower relations for simple come from Princeton University’s WordNet. Full sources and licences.