"The author explains his book."
Translation:Författaren förklarar sin bok.
I thought that it has to be sin if the possesser is the subject of the sentence. Why is hans correct?
Because the English sentence is ambiguous. He might in fact explain someone else’s book from the information we’re given. Therefore, we have to accept both.
If you get it as a multiple choice question, you can get shown correct options with either sin, hans, or both. It's selected randomly.