Il means the, if it was "Io scrivo un libro" then you could say "I write a book"
normally we say i write a book not the book because the book still not known so it is a book not the book
One would never state in English that "i write the book". This is a classic example of grammar vs. use. If someone told me that i would assume they are esl. Albeit translating grammar is not a matter of apples abd apples... Sometimes a guava gets thrown in.
I am really having hard time to understand il and un :/ . They speak too fast
I put the correct answer: lo scrivo il libro. I write the book. But it said i was wrong.