"You write the book."
Translation:Tu scrivi il libro.
Only by context. In the beginning sentences, we had only learned "you singular". So, it had to be "tu". In this sentence, I had to choose from a selection of words and the only correct choice was "tu scrivi". Once we learn both "tu" and "voi", Duo will give credit for either, if it is not made clear it wants one over the other.