sain / sane / seine are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
sain: NA
sane: (adjective) mentally healthy; free from mental disorder.
seine: (noun) a French river that flows through the heart of Paris and then northward into the English Channel. (noun) a large fishnet that hangs vertically, with floats at the top and weights at the bottom. (verb) fish with a seine; catch fish with a seine.
sain: s·EY·n
sane: s·EY·n
seine: s·EY·n·iy
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)