I really don't understand this construction, and the English sentence is not terribly common. My first question is: Is this the most idiomatic way to say, "Your room is dirty" in Polish? Secondly, why use the adverbial "brudno" and not the adjectival "brudny"? Thirdly, would it be unidiomatic for the word order to be: "Jest brudno w twoim pokoju?" Many thanks for any help.
We had a bit of a discussion on this topic, and we decided to accept "Twój pokój jest brudny", which corresponds to "Your room is dirty". But we had some doubt about it, so it's not like it's such a great option. We were also discussing "Masz brudny pokój", there was even more doubt, but we're going to accept it as well.
The adverb is used because in this sentence there isn't really any subject. It is constructed similarly to, e.g. "Jest zimno". (It is cold.) I'm not sure about English, but in Polish there's no subject here.
"Jest brudno w twoim pokoju" works.
Frankly, my first choice in natural language would just be to say "Masz bałagan w pokoju", which would translate to "You've got a mess in your room". But I think that's too far from the English sentence to be accepted here.
From my understanding "Its" is the possessive form of "It", as is "Do you remember my old phone? Its screen broke last week"; whereas "It's" is the short form of "It is", as in "Do you remember my old phone? It's so crappy that a thief refused to steal it" I understand it may sound too harsh for native speakers but it really sounds bad and/or confusing for a non-native speaker
That's quite a common change. It's actually something that Polish pupils learn at school. I mean, when you learn Polish, you usually learn the words written (especially here), so our 'students' don't have problems with orthography. For a young native, it is not that obvious whether a new word they heard is written with "u" or "ó" (which after all sound exactly the same). So the rule is that generally if the "ó/u" sound changes into "o" with declension, that means that it's written with "ó". Of course, there are exceptions (surprise!), but generally they're not very common.