`1,` When the police entered the room, everyone was playing cards happily.

`2,` If he had enough money now, he could buy a new pocket calculator.

- Câu điều kiện loại 2 (không có thật ở hiện tại) : If + S + V (quá khứ đơn), S + would/ could + (not) + V nguyên thể

`3,` While we were sleeping in the bedroom, there was a knock on the door.

`4,` I witnessed an accident on the road while I was driving to the office in the morning.

`5,` The man who is standing at the gate now is my friend's father.

- who là ĐTQH thay thế cho danh từ chỉ người

- now `->` Thì hiện tại tiếp diễn

`6,` The doctor came into the room when the patients were sleeping soundly.

`7,` While I was listening to the news on the radio, someone called my name.

`---------`

`**` Trong quá khứ :

- Hành động đang diễn ra chia thì quá khứ tiếp diễn :

`( + )` S + was/ were + V-ing

`(-)` S + was/were + not + V-ing

`(?)` (WH-word) + was/ were + S + V-ing?

* Note :

- I/ She/ He/ It/ Đại từ bất định/ Danh từ số ít/ Danh từ không đếm được : was

- You/ We/ They/ Danh từ số nhiều : were

- Hành động xen vào chia thì quá khứ đơn :

`(+)` S + Ved/ V2

`(-)` S + did not/ didn't + V (bare)

`(?)` (WH-word) + did + S + V (bare) ?

`@` To be :

`(+)`S + was/were 

`(-)`S + was/were + not

`(?)` (WH-word) + was/were + S + O?