"I write a book."
Translation:Io scrivo un libro.
Did you type "Lo", "lo" or "Io"?
Be careful with that. It's a common beginner's mistake due to the font type used which shows upper case "i" that looks like lower case "l".
write = scrivo
a book = un libro
the book = il libro
The book = Il libro
Notice how similar the upper case "i" is to the lower case "l".
Happy e-learning on Duolingo.