8. This man (to be) a writer. He (to write) books. He (to write) books since he was a young man. He already (to write) eight books.

8. This man is a writer. He writes books. He has been writing books since he was a young man. He already has written eight books.

Объяснение:

1. 'This man is a writer' — общее утверждение о роде занятий.
2. 'He writes books' — это факт, регулярное действие.
3. 'He has been writing books since he was a young man' — действие началось в прошлом (с молодости) и продолжается до сих пор, поэтому Present Perfect Continuous.
4. 'He already has written eight books' — действие завершено, и важен результат (8 книг), поэтому Present Perfect Simple.