6, If the boy didn't work too hard, He wouldn't be sick.
7, The trees will die if we don't water them every day. 
8, Can you play the guitar when you were young? 
9, It’s the first time I have seen her since 2018.
10, Hoa has not visited her parents since she left her house.
11, There will be less pollution if people cycle more.
12, He often goes swimming with his friends on Sunday afternoon
13, My cat is prettier than yours
14, These shoes are the most comfortable in the shop
15, She is traveling on a plane for the first time