1. Maggie went swimming twice a week.
S+ Vqk+...
2. They didn't usually eat meat.
S+ didn't+ adv+ Vqk
3. While they were swimming, they saw a shark.
While + S+ was/ were+ Ving, S+ Vqk
( diễn tả một hành động đang xảy ra thì một hành động khác xen vào)