"I write a book."
Translation:저는 책을 씁니다.
the first is more like "take note, note down, jot down" it doesn't require much thought, e.g. to-do entry, phone number, a math answer from the answer sheet. the second is more "hand write, write prettily, creatively write" which requires effort and thought, e.g. a letter, a song, a essay answer that requires you to think creatively.