21. I didn't see the signal, so I didn't stop.

→ If I had seen the signal , I would have stopped 

Câu đk loại 3: If + S + had + Vpp + O , S + would have / could have + Vpp + O 

23. She didn't know you were in hospital, so she didn't visit you.

→ If she had known you were in hospital, she would have visited you

Câu đk loại 3: If + S + had + Vpp + O , S + would have / could have + Vpp + O 

25. She is very shy; that's why she doesn't enjoy parties.

If she weren't shy, she would enjoy parties.

Câu đk loại 2=> ko có thật so với hiện tại: If+ S+ were+ adj/N, S+ would V

26. He doesn't take exercise; that's why he is so unhealthy.

If he took exercise, he wouldn't be unhealthy

Câu đk loại 2=> ko có thật so với hiện tại: If+ S+ Vqkđ, S+ would V

27.I haven't the right changes so we can't get tickets from the mach

If I had the right changes, we could get tickets from the mach

Câu đk loại 2=> ko có thật so với hiện tại: If+ S+ Vqkđ, S+ would V

28. The weather is cold today, so we won't go swimming.

→ If the weather weren't cold today, we would go swimming.

Câu đk loại 2=> ko có thật so với hiện tại: If+ S+ were+ adj/N, S+ would V

29. Carol didn't answer the phone because she was studying.

If Carol hadn't been studying, she would have answered the phone

Câu đk loại 3: If + S + had + Vpp + O , S + would have / could have + Vpp + O 

30. We don't have a map. That's why we can't find the street.

If we had a map, we could find the street.