1. When I was listening to music, I heard the doorbell. 
2. His boss went into the room while he was typing a letter.
3. Anne was riding her bike to school yesterday when Peter saw her.
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@ hành động đang xảy ra thì một hành động khác chen ngang -> thì QKTD
When + S + was/were + V-ing, S + Ved/V2
While + S + was/were + V-ing, S + Ved/V2
When + S + Ved/V2, S + was/were + V-ing

1. I was listening to music . I heard the doorbell.-> When I was listening to music, I heard the doorbell.
2. He was typing a letter when his boss went into the room.-> His boss went into the room when he was typing a letter.
3. Peter saw Anne riding her bike to school yesterday -> Anne was riding her bike to school when Peter saw her yesterday. Công thức sự phối hợp thì qktd và qkđ, mệnh đề thời gian.   Đang làm công việc này (was/were V-ing) thì công việc khác xen vào (V-ed).