Đáp án+Giải thích các bước giải:
`1,` Last week we had to do three tests. 
lỗi sai:  had to

Sửa: Last week we had three tests. 
`=>` "Had" tự nhiên hơn, không cần "had to do" 
`2,` When we  were travelling to Cambridge, the car broke down 

lỗi sai: were travelling
Sửa: While we were travelling to Cambridge, the car broke down. 

`=>` Dùng "while" để diễn tả hai hành động cùng lúc.
`3,` I think I'm going to visit your house 
lỗi sai: I'm going 

Sửa: I think I'll visit your house. 
`=>` "I think" `→` dùng "will" cho quyết định ngay lúc nói, không dùng "going to".
`4,` When I was younger, I spent a lot of money on sweets
lỗi sai: a lot of

Sửa: When I was younger, I used to spend a lot of money on sweets. 
`=>` "Used to" diễn tả thói quen quá khứ, phù hợp hơn "spent".