"He eats his apple."
Translation:Han äter sitt äpple.
Hans vs sitt example: 1. Patrik kysser sin fru. 2. Patrik kysser hans fru. In English both sentences translates to “Patrik is kissing his wife”. In Swedish however, you make a distinction between “his own wife” = sin, and his as in someoneelse’s wife. - source: http://www.thelocal.se/blogs/theswedishteacher/2010/03/25/sin-sitt-sina/
en/ett, sin/sitt : the more you read and hear the language the easier it getr to just feel which one to use. In German and French and so on, it is the same -- or worre I would say -- and I never learned any rules (in that area, there are so many much more ''meaningful'' ones). just my two cents