soft has 17 distinct synonyms spread across 12 of its 20 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 3 recorded opposites are hard, loud, hardened.
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.
yielding readily to pressure or weight
compassionate and kind; conciliatory
“he was soft on his children”
(of sound) relatively low in volume
“soft voices”
not protected against attack (especially by nuclear weapons)
“soft targets”
(used chiefly as a direction or description in music) soft; in a quiet, subdued tone
“the piano passages in the composition”
(of speech sounds); produced with the back of the tongue raised toward the hard palate; characterized by a hissing or hushing sound (as `s' and `sh')
3 of the 20 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 soft.
Synonyms, antonyms and the broader and narrower relations for soft come from Princeton University’s WordNet. Full sources and licences.