i was gonna ask this, and you just happened to have provided the info, though no one has asked yet xD
Nor do I. Duolingo doesn't seem to be very good with the grammatical side of things. They just expect you to pick everything up as you go along.
I discovered that if you use duolingo on their website, they give really helpful grammatical explainations before a lot of the lessons. I went back and read all the lessons after I found out. It makes SO much more sense now!
Duolingo is inconsistent in enforcing punctuation. After getting correct answers several times without the terminal punctuation mark, it turns around and rejects the correct answer on this, presumably due to the lack of the period at the end (the ONLY difference between my answer and theirs).
I don't mind if they enforce punctuation, they really should, but it would be nice if they would do consistently.
Would you not rather, or at least is it not possible, to say "Es sind drei Bücher." ?:)
I think it would be "Sie sind drei Bücher" if you are talking about where the books are, not just that they generally exist
Why in the world not "there is three books"?!?!?!?!??! it was even in the hints!