"The sour apple tastes bad."
Translation:신 사과가 맛없어요.
In this case, it's the adjective form of 시다 "to be sour": 시 + ㄴ/은 = 신. Homonyms, brought to you by natural language evolution. :D
Yeah, and it also means monkey on the Chinese Zodiac. I would never say an apple is sour in English anyway because people say it's wrong; I'd call it (too/very) tart ((너무/아주많이)산미있는/시큼한). I've heard people describe them in English (unripe) or Japanese (aogusai = "stink of green") as the equivalent of 미숙한 (immature). A dictionary says 산미있는 means sour, but I learned that as tart in Japanese, and they prefer that to suppai, which does mean sour, in these cases . . .