Свој - the reflexive possessive

Use свој whenever the possessor is the same person as the subject of the clause, in any grammatical person - not only the third. Марко воли своју кућу. means Marko loves his own house; Марко воли његову кућу. would mean Marko loves some other man's house.

Свој declines exactly like мој/твој and, being reflexive, never has its own separate first- or second-person form - Ја читам своју књигу. (I'm reading my own book) is just as correct, and just as common, as using моју there.

Чији? - asking whose

Чији/чија/чије (whose) is the question word that possessive pronouns answer, and - like који/која/које in questions-a1 - it agrees with the gender and number of the thing possessed, not with whoever might own it.