Mum cooked / was cooking dinner when someone knocked/was knocking on the door. sue was

Mum was cooking dinner when someone knocked on the door. - Мама (что делала?) готовила обед, когда кто то (что сделал?) позвонил в дверь.

Was cooking - форма Past Continuous Tense от глагола to cook;

Knocked - форма Past Simple Tense от глагола to cook.