Questions & Answers about Är det din bok? Nej, den är hans.
Why do we use det to ask the question, but den to answer it?
When pointing something out or asking 'is that...', Swedish always uses the generic det (that/it) regardless of the noun's gender. But once the noun bok (an en-word) is established, we must use den to refer back to it.
Why does the second sentence use hans instead of sin?
The reflexive possessive sin is only used when the owner is the subject of the same clause. Here, the subject of the sentence is den (the book), not the person who owns it, so you must use hans.
Does hans need to agree with the noun's gender like din does?
No, third-person possessive words like hans (his), hennes (her), and deras (their) never change their forms. Words like min and din change depending on the gender and number of the noun, but hans stays exactly the same.
English has 'your' vs 'yours'. Does Swedish have separate words for when the possessive stands alone?
No, Swedish uses the exact same possessive words whether they are attached to a noun or standing completely alone. So din means both 'your' and 'yours', and hans works perfectly for both 'his book' and 'the book is his'.