"He went to his sister's room."
Translation:Šel do sestřina pokoje.
"Místnost" is any room, even a utility room, a closet, a kitchen. "Pokoj" is a room where you live, a living room, a bedroom or a hotel room.
Místnost is the general word for 'a room', pokoj is a room where you live.
Living room is (obývací) pokoj, hotel room is pokoj, bedroom is either ložnice or pokoj.
Bathrooms, classrooms, utility rooms, kitchens etc. are místnosti, not pokoje.
Why it is "sestřina"? I mean, we say Kateřinin pokoj, right? Then why not "sestřin"? Thanks a lot!
…and “Kateřinin” is a possessive adjective, which is usually formed from proper names; the rules for other nouns seem to be far from simple. I cannot tell you why sestra does not have it; bratr, for instance, does (bratrův).
You often find possessive adjectives in Czech place names, such as Karlův most.