第一句用while,因为两个动作都是持续性的:My mother was watching TV while I was doing my homework.
第二句用when,因为敲门是个短暂的动作:I was watching TV when he knocked the door.
第一句用while,因为两个动作都是持续性的:My mother was watching TV while I was doing my homework.
第二句用when,因为敲门是个短暂的动作:I was watching TV when he knocked the door.