In German, you use inversion when asking a yes/no question. That is, you switch the position of the verb and pronoun. You are actually doing the same thing in English. They are (pronoun, verb), are they? (verb, pronoun). Sie schreiben (they are writing/they write), schreiben sie? (are they writing?/do they write?).
I wrote "Schreiben Sie ein Buch?" , forgetting that until now we didn't learn the polite form of you in Duolingo. Without the context "Is the question asked to a person we just met?"Are you writing a book?" Or is the question asked about "they" like in "Are they writing a book?" impossible to find out! Duolingo did not notice my capital S. So no harm done...