S + was/were + V-ing
Background actions; actions in progress interrupted by a past action.
Often used with when/while; set the scene.
I was cooking when he called.
Tôi đang nấu ăn thì anh ấy gọi.
They were playing football while it was raining.
Họ đang chơi bóng khi trời đang mưa.