"Zij lezen de boeken."
Translation:They read the books.
I don't know If you are using Duo on a computer or Tablet but if you are on a computer if you click on tips you'll see that on this lesson we only see plurals where you have to add "EN". Also we only add "S" when you have a noun that ends with A SINGLE vowel. You add: ' + S : or just S if it finishes with and E.