I don't understand the differences between hans, honom and sin/sitt/sina (I do understand sin is for en-words, sitt for ett-words and sina for plural). Can someone please tell me when you have to use what? I have the same problem with henne, hennes and sin/sitt/sina.
Honom and henne are the same as "him" and "her" in English, for example: Han/hon ser mig - He/she sees me vs. Jag ser honom/henne - I see him/her. Hans and hennes are the possessive pronouns for han and hon respectively, equivalent to "his" and "her" in English. Sin/sitt/sina is also a possessive third person pronoun, but used for something possessed by the subject; ex. Han tvättar sin hund - He washes his (own) dog vs. Han tvättar hans hund - He washes his (not his own) dog. If you're still confused I'd recommend looking at the tips and notes for the possessives skill on the Swedish course, which does a better job of explaining it than I can: https://www.duolingo.com/skill/sv/Possessives
From my understanding, the basic difference between "sin" and "hans/hons" has to do with whether or not the owner of the object is interacting with it or not. If they aren't interacting with it, it's "hans/hons" and if they are it's "sin".