"Tutaj" and other pronouns https://pl.wiktionary.org/wiki/Aneks:J%C4%99zyk_polski_-_zaimki in their long forms are generally used before the verb. "Tu" and other pronouns in short form are used rather after the verb.
Although Duo knows many apostrophised English forms, it graded my "Why's it dirty here?" as 'Correct' whilst stating that "Why's" is 'missing a space'. I've often had similar responses from Duo when I've used common apostrophised English forms (i.e. ones that Duo knows) in the middle of a sentence.
"Brudno" is an adverb in Polish - many adverbs in Polish do end either with -o or with -ie.
In the English sentence you use an adjective, but usually when there is a dummy subject in English sentence ("it" or "there") - in Polish sentence you skip the dummy subject and replace the adjective with an adverb. Compare:
- Your room is dirty. (subject: room; adjective: dirty) = Twój pokój jest brudny. (subject: pokój; adjective: brudny) - there is no dummy subject in this sentence, because "your room" is a real subject.
- It is dirty in your room. (subject: it; adjective: dirty) = W twoim pokoju jest brudno. (subject: none; adverb: brudno) - in the English sentence there is a dummy subject (1), and "your room" is only an adverbial of place; the adjective "dirty" does not really describe "it" (the subject), but the general look of the room, or the mess that is there. In that case the Polish sentence does not need a real subject, there is only a "logical subject" (you can imagine what the adverb refers to, but it is not mentioned in the sentence).
- Why is the room dirty? (subject: the room; adjective: dirty) = Dlaczego pokój jest brudny? (subject: pokój; adjective: brudny)
- Why is it dirty here? - (subject: it; adjective: dirty) = Dlaczego tutaj jest brudno? (subject: none; adverb: brudno) - again "it" is not a real subject, but just a dummy one, and "here" is an adverbial of place.
(1) "it" is not a real subject if you cannot substitute "it" with "this" or "that".