For example：They were playing when Dad came home.
was / were ….. verb (動詞) +ing
(3) 「簡單過去式」和「過去進行式」常會連在一起使用，我們會用 "when" 或 "while" 把句子中的兩個動作連在一起。
when + 短動作 (簡單過去式)
while + 長動作 (過去進行式)
For example 例如：
We were having dinner when Sam came.
While we were having dinner, Sam came.
The team members were playing basketball yesterday after school.
Was Miss Chan working at school from 7pm to 8pm last night?
Paul was not listening to the teacher at all in the last lesson.