Breakdown of Mój współlokator robi trochę bałaganu w swoim pokoju.
robić
to make, to do
mój
my
w
in / on (a day)
pokój
room
Questions & Answers about Mój współlokator robi trochę bałaganu w swoim pokoju.
Why is swoim used here instead of jego?
The reflexive possessive pronoun swój means 'one's own'. It is used when the owner of the object is also the subject of the sentence. Since the flatmate (the subject) is making a mess in his own room, we use swoim. If we used jego (his), it would imply he is making a mess in some other guy's room.
What case is swoim pokoju in, and why?
It is in the locative case. The preposition w (in) takes the locative case when describing a location or state of rest. Because pokój (room) is masculine, its locative form is pokoju, and the matching oblique form for swój is swoim.
If swój means 'one's own', why don't we say Swój współlokator at the beginning?
Swój must always refer back to the subject of the sentence. This means it can never be part of the subject itself! Here, the flatmate is the subject performing the action, so we must use the regular possessive pronoun mój.
Why does the word bałagan (mess) change to bałaganu here?
It changes to the genitive case because it comes directly after the quantifier trochę (a little). Words expressing quantity, like trochę, dużo (a lot), or mało (few/little), always force the following noun into the genitive case.