10. His father hasn't smoked since was seriously ill

Công thức chuyển từ thì QKĐ sang HTHT:

S + V2 + thời gian + ago

→ S + have/has + V3 + for + thời gian

11. His sisters had stopped eating too much fast food after they had trouble losing weight.

Công thức thì quá khứ hoàn thành: 

S + had + V3 + before/after/..

12. They have taken part in the charity since they were grade 10th

Công thức:

S + started / began + V-ing / to-V 

→ S + have/has PII + since/for + ...