In the middle of our dinner , someone knocked at the door. → While

While we were having our lunch, someone knocked at the door

`=>` While + S + V(QKTD)…,S + V(QKD).

`=>` Trong khi chúng tôi ăn trưa, có tiếng gõ cửa.

`=>` QKTD: S + was/were + V-ing (+ O)

`=>` QKD: S Ved/2