12. If you don't return this book to the library today, you'll have to pay a fine.

⇒  Unless John returns this book to the library, he’ll have to pay a fine.

Cấu trúc: Unless + S + V (simple present), S + will/can/shall + V

13. If I didn't know the number, I would not ring her up.  (Câu đk loại 1)

⇒ Unless I knew the number, I would not ring her up.

Cấu trúc: Unless + S +Ved / V2 (simple past), S+ would + V (Câu đk loại 2)

14. If Peter didn't study hard, he could not get good marks.

⇒ Unless Peter studied hard, he could not get good marks.

Cấu trúc: Unless + S +Ved / V2 (simple past), S+ would + V (Câu đk loại 2)