=>
1. had been -> were (câu đk loại 2 If S V2/ed, S would V)
2. Unless -> If (unless = if not)
3. knew -> known (câu đk loại 3 If S had V3/ed, S would have V3/ed)
4. am -> were (câu đk loại 2)
5. in -> out (find out : tìm ra)
6. would stop -> would have stopped ( câu đk loại 3)
7. you would -> would you ( nghi vấn -> đảo would lên trước chủ ngữ)
8. is -> were (câu đk loại 3 -> không có thật ở quá khứ)
9. If -> x (đảo ngữ đk loại 3 Had S V3/ed, S would have V3/ed)
10. will -> would (đk loại 2)
11. be -> been (đk loại 3)
12. because -> if ( đk loại 2)
13. would go -> will go (đk loại 1)
14. will -> x (đk loại 1)
15. to -> x (đk loại 2)
16. to -> for (apply for job : xin việc)
17. occur -> would occur (đk loại 2)
18. would have studied -> had studied (đk loại 3)
19. will be -> is (đk loại 1)
20. saw -> see (đk loại 1)