I have been using Duolingo since I first saw this post, but frankly their German course is appalling.
They write their English sentences to be translated into German in such slangy American English that at times they are incomprehensible, they expect you to translate the simple past tense to the German perfect tense, even in instances where no native German speaker would, a good many of their listening exercises are so inaudable that you cannot avoid making mistakes, and they use slangy German expressions quite often in preference to more correct German.
I find these things infuriating, and there is no way you can complain to whoever runs duolingo that I can find.
Most maddening of all, they adore sentences such as "When are you leaving?" and only accept one out of three possible German tranlations. As there is no reading exercise or any other form of context "you" in this kind of sentence could mean either "du" singular informal, "ihr" plural informal, or "Sie" singular and plural formal German pronouns.
I put up with this as I am using it to revise my knowledge of German, but as they never explain grammar at all, I doubt you could actually learn the language properly using duolingo.
And they insist upon telling us what else we could have written in English - rather unnecessarily as it is German we are trying to learn!