"Where are you?"
Translation:Var är du?
Are du and ni used in different contexts? or are they always interchangeable?
Du/dig is singular, ni/er is plural. Compare du/dig to thou/thee in English to illustrate the difference.
So, no, they're not interchangeable.
Thou/you would actually be a familiar/formal distinction like du/Sie in German. You're looking for you(du)/ye or y'all(ni).
As I know, in German the plural "you" is "ihr" and the polite singular or plural "you" is "Sie". Is "ni" used also as the polite form of "you" in Swedish in the same way as "vous" in French?
Is "är" the only verb that you don't switch the noun and the verb to for a question?
It isn't necessarily that the words are switched, the thing is that in questions, the verb goes before the subject (whereas in non-questions, the verb is in second place). And the only thing that can go before the verb is question words, like var here.