"What street does he live on?"
Translation:На какой улице он живёт?
Can someone explain to me the use of какой here? I understand that it's a declension of как, but why is it being used here instead of что or какая?
Какой is different from как, which means 'how', and which is not declined. Какой, on the other-hand means 'which' and is declined same as adjectives. Therefore, as with adjectives, the feminine endings, save the nominative and accusative case, are always -ой. This means that these feminine forms, the prepositional case required here included, will be identical with the masculine nominative form 'какой'.
I hope I could help those who have been wondering the same.
You'll be understood, but the word order is rather unnatural. You would only use it to show disbelief about his street: He lives on what street?
Thank you. The order of words in Russian seems really complicated to me. Duolingo often tells me that my answer is wrong and I don't know why. I am never sure if I should report such borderline cases or not. =/
The funny thing is that in this example the word order (aside from "does", which is not used in Russian, & a preposition at the end) is basically the same in English & Russian. And you can certainly move the preposition to the front of the sentence in English, which would make the word orders match even better.