1. When I saw them, they were waiting for the bus yesterday. 

2. While she was cleaning the floor, she broke the flower vase. 

3. The men had been playing cards for 3 hourse before I came.

4. Yesterday, my father was reading newspaper while my mother was talking on the phone with my aunt. 

5. The boy was running down the stairs when he failed. 

6. When John and I got to the theater, the movie had already started. 

1. was waiting

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

2. broke

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

3. had played, came

QKHT + before + QKĐ

4. was reading, was talking

2 hành động cùng xảy ra trong quá khứ → QKTD

5. was running, fell

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

6. got, had already start

Hành động trong quá khứ → QKĐ

Hành động trước 1 hành động trong quá khứ → QKHT