Sin if the thing you're referring to is an en-word, sitt if it's an ett-word:

• Mannen älskar sin hund. (Because it's en hund)
• Mannen älskar sitt hus. (Because it's ett hus)

Try this explanation also:

https://www.reddit.com/r/Svenska/comments/3qitpu/what_is_the_difference_between_dindittdina_and/cwfnh08

Context. "sin" if it's his own dog, "hans" if it's some other male person's dog. In this case, translating a single sentence where the English language is ambiguous, either translation should be fine. But some native Swedes would mess it up too, even if they knew whose dog it was! :)

The reflexive possessive pronoun "sin" is used if "He loves his (own) dog" vs the possessive pronoun "hans" which would mean "He loves his (another guy's) dog".

Sin -> his dog (he has only one dog) Sina -> his dogs (he has several dogs)

Ett and En are both used for singular forms. But it is just like A or An.

But there is not really a difference, you learn as you use them more. But to make it easier, think of how it sounds.

Ett katt, sounds wrong right? Because it is supposed to be En katt (A cat). If you try to memorize what sounds off and what sounds correct, you'll get the hang of it.

Examples with En: En kvinna, en man, en hund, en katt, en cykel, en pojke, en flicka, en klänning.

Examples with Ett: Ett hus, ett äpple, ett arbete, ett hem.

Try to memorize which is used for what and which sounds the best and the least odd to you. Goodluck!