"Are they writing a book?"
Translation:Schreiben sie ein Buch?
You don't need "Sind". All verbs I know of in German do not need "Are" "Is" "Am". You can tell with the end of the verb. "Schreiben" ends with "en" so it works with "We" and "They". You don't need "are" because the words do it for you. When asking a question, the format is different from a statement. The verb comes first.
So, I wrongly wrote: "Schreiben ein Buch sie?"
Duolingo corrected me with this: You used the wrong word. "Schreiben sie 1 Buch?"
(That is a cut and paste...so, yes, the number 1 is being suggested for ein, I guess. I don't think Duo got the answer any more correct then I did this 1 time.)