"You are drinking its water."
Translation:Du drikker dens vand.
From your name I would guess you are a native Spanish speaker. I think Spanish is like French, where it is the gender of the object possessed that is important, not the gender of the possesser. Danish is like English, so the gender of the possesser determines the pronoun.
OK, why is it not "I drikker dens vand." The English is usually ambiguous about the number of persons referred to by you, but Danish isn't. That means, surely, that unless the word "you" is qualified (eg as in "you people" or "you all") it can be translated into Danish as du or I.
Or am I missing something?
Because sin/sit/sine is only used when the subject is 3rd person singular, and the subject is drinking their own water (like in: "he is drinking his (own) water" = "han drikker sit vand"). But here, the subject is 2nd person and the water belongs to someone else, thus none of the two obligatory conditions is fulfilled.