"You have a really quiet room."
Translation:Sinulla on todella hiljainen huone.
'Room' is definitely the object of the English sentence, but I've seen it said that huone is the subject of the Finnish sentence. Very literally, "At/On you is a really quiet room".
Compare this with Tuolla on todella hiljainen huone "Over there (At that place) is a really quiet room/A really quiet room is over there".
But the object isn't always partitive. Sometimes it's accusative, as in Hän maalaat sinut "He will paint you". Sometimes the object looks nominative, like Syö omena! "Eat the apple!"
And then the subject is sometimes partitive, particularly with uncountable nouns, like Pöydällä on ruokaa "On the table is food/Some food is on the table".