4, chọn C 

A. although i paid attention to .... --> sai nghĩa

B. I would ave understood the lesson... --> sai nghĩa

D. unless i failed to understand ... --> sai nghĩa 

5, chọn C 

A. had ...., I couldn't make .... --> sai cấu trúc câu điều điện loại 3

Cấu trúc câu đk loại 3 : If + S + had + V qkpt, S + would/ could/... + have + V qkpt 

B. if my tutor didn't help ... --> sai cấu trúc

D. If..., I could have made --> sai nghĩa 

6, chọn B

A. but for..., we would have ... --> sai nghĩa 

C. without ..., we could have ... --> sai nghĩa 

D.  if we weren't wearing ... --> sai cấu trúc câu đk loại 3

7, chọn C

A. if it snowed heavily .... --> sai cấu trúc

B. had it not snowed heavily, they would have... --> sai nghĩa 

D. if it didn't snow, ... --> sai cấu trúc

8, chọn B

A, had it not ..., he wouldn't be succesful ...--> sai cấu trúc

C. but for...., he wouldn't be successful ..--> sai cấu trúc

D. without his ..., he would have been ...--> sai nghĩa

9, chọn D 

A. without ..., they wouldn't ... --> sai nghĩa

B. if it hadn't..., they would have been ...--> sai nghĩa

C. if it rained ...., they wouldn't ...--> sai cấu trúc