过去进行时
——表示过去正在进行的动作，经常用在when, while, as引导的状语从句中。
结构：was/were+doing
When my husband was going into the dining room this morning, he dropped some coins on the floor.
Their father was watching TV while they were having dinner.
★变疑问句将be动词移到句首
Was their father watching TV while they were having dinner.
★变否定句在be动词后面加 not
Their father was not watching TV while they were having dinner.