1. People suggest exhaust fumes from vehicles should be reduced.

2. My mom suggested keeping the surroundings clean and green.

3. I suggest visitors should stop littering in Phong Nha Cave.

4. My friends suggest visiting the Royal Citadel when being in Hue.

Suggest + V-ing 
Suggest + that + S + should + V 

`7.` People suggest the exhaust fumes from vehicles should be reduced.

`-` BĐ ĐTKT: S `+` ĐTKT(should/must/can/...) `+` be `+` PII `+` (by O)

`8.` My mom suggested keeping the surroundings clean and green.

`->` Keep sb/sth adj `:` Giữ cho ai đó/cái gì nhưu thế nào ...

`9.` I suggest that visitors (should) stop littering in Phong Nha Cave.

`10.` My friends suggest visiting the Royal Citadel when being in Hue.

`------------------`

`-` Cấu trúc: S `+` suggest (chia theo thì) `+` V-ing `:` Ai đó gợi ý làm gì ...

`-` S `+` suggest (chia theo thì) `+` S `+` (should) `+` V-inf 

`->` Ai đó gợi ý ai (nên) làm gì ...