When someone asks a question along the lines:
Чей это кот? Чья это собака? Чьё это письмо? Чьи это очки?
I have the following two questions regarding the structure of these interrogative sentences:
1. What type of pronoun is чья/чей/чьё/чьи here? Is it a pronoun which replaces a noun, or a pronoun that replaces an adjective?
For example - "This is your car" -- here "your" takes the place of an adjective
"This car is yours" -- here "yours" takes the place of noun
So in these questions, does чьё/чья/чей/чьи mean "whose" in the sense of "Whose is this?" (replacing a noun) or "Whose car is this?" (replacing an adjective)?
2. I have an essentially analogous question about the function of "это" in this type of question.
In these questions, does "это" mean "this" in the sense of "Think about this" (takes the place of a noun) or "this" in the sense of "Think about this question" (takes the place of an adjective)?
My confusion is as follows: чей/чья/чьи/чьё declines with the noun in "...", but this isn't enough to tell us whether it is taking the role of a noun or adjective, because both of these types of pronouns decline the same in Russian -- i.e. "(Это) авто -- твоё." and "Это -- твоё авто." are both valid sentences, and "твоя" is declined the same way in both, despite the fact that it serves a different function in both.
But in German, one would have "Das Auto ist deins" and "Das ist dein Auto". In German one can tell from the declension of "dein" what function it is serving in each sentence, and from that infer what the function of "das" is (along with whether it is separated from the noun by the copular verb, which generally isn't used in Russian).
The only hint in Russian that allows one to differentiate between "Это авто твоё" and "Это твоё авто" (at least as far as I can tell) is the word order. But the rules for word order in questions are usually different from the rules for word order in regular sentences, so I really don't know what at all to make of "Чьё это авто". Hence any help would be greatly appreciated.