"Sir, are you writing a book?"
Translation:Señor, ¿escribe usted un libro?
In English, we can use the present continuous tense to talk about activities that are on-going, but that we aren't necessarily doing at that moment. However, in Spanish, this is not the case. In Spanish the present continuous is reserved for whatever action you are completing in that precise moment. For example, in English, it would be common to say "I am working at the supermarket," to one of your friends when you are not actually on the clock. But, if you are speaking Spanish and you say, "Estoy trabajando en el supermercado," that would actually mean that you are on the clock working at that moment. Check out LightSpeedSpanish on youtube - they have some great videos explaining grammar concepts like this. Their videos are very succint and to the point - about 10 minutes each!
Despite Tessa Jade's comment below, she is attributing context to a situation that lacks context. We have only the translation to work with. If the English asks for present continuous, the correct Spanish response should also require present continuous tense. This confuses people who are doing our best to learn difficult verb tensing. DUOLINGO NEEDS TO CORRECT THIS ERROR.
Present tense (indicative) in Spanish means three things.
Yo hablo inglés: I speak English. I do speak English. I am speaking English.
Yo como pan: I eat bread. I do eat bread. I am eating bread.
Yo vivo en Buenos Aires: I live in Buenos Aires. I do live in Buenos Aires.