In Danish, "my" & "mine" is either min, mit, or mine, the differences are:
min = when the following word uses -en : The dog is eating my food = Hunden spiser min mad. (mad is an -en word, as you'd say maden for 'the food')
mit = when the thing that's owned is a -et word : You are eating my apple = Du spiser mit aeble (mit is an -et word, as you'd say aeblet for 'the apple')
mine = when there is a plural of the thing being owned : I have my horses = Jeg har mine heste (But if you were saying "I have my horse" - just the one horse- it would be "Jeg har min hest")
in duolingo there is need of grammar, otherwise we have to read all the comments hoping to find an explanation to the grammatical rules.
"Sin" is for singular -n words, and "Sit" is for singular -t words:
"De spiser sin ost" = They eat their (own) cheese.
"Hun spiser sit brød" = She eats her (own) bread.
"Sine" is for both -n and -t plural words:
"Han spiser sine appelsiner" - He eats his (own) oranges.
"Katten spiser sine æbler" - The cat eats his (own) apples.
where is the difference between "sin" and "hans" and when do I have to use which one?
"Sin" means that he is reading his own newspaper. You would use "hans" if he is reading another man's newspaper.
Hans and Hendes are just the same as his and hers. Sin follows more of a "there" but if the sentence starts with he or she we know we would have to put his or hers.