"Isn't it dirty?"
When do I use きたない and when きたなく? In another discussion somebody wrote that one of them is the adjective form while the other one is the adverb, but the sentence "It is dirty" uses きたない while the negation is translated as きたなく. This would imply that everything I have learned about adjectives and adverbs is completely nonsense...
Some extra info regarding on why negative form is used in questions in Japanese. Shortly, to be more polite.
Tae Kim (p. 219 / 5.1.3 Using passive form to show politeness ):
While we will go over various types of grammar that express a politeness level above the normal -masu/- desu forms in the next lesson, it is useful to know that using passive form is another more polite way to express an action. In Japanese, a sentence is usually more polite when it is less direct. For example, it is more polite to refer to someone by his or her name and not by the direct pronoun "you". It is also more polite to ask a negative question than a positive one. (For example, 「しますか？」 vs. 「しませんか？」 ) In a similar sense, using the passive form makes the sentence less direct because the subject does not directly perform the
汚いですか？ less polite
汚くないですか？ more polite