(Công thức trong ảnh nha)
Note: Hầu hết các câu trong đề bài đều áp dụng cách dùng "Thì QKTD diễn tả hành động đã đang xảy ra trong quá khứ thì một hành động khác xảy đến, hành động xen vào chia thì QKĐ" của thì QKTD
11. saw - were trying
12. was walking - felt
13. were driving - appeared
14. was she driving - happened
15. was waiting - came
16. invited - was visiting
17. didn't want - was waiting
18. rang - was watching
19. began - was sitting
20. went - were having