表示过去某个时间正在发生的动作。
常用的时间状语有：at this time yesterday、at that time、this morning、the whole morning、all day yesterday、from nie to ten last evening、when、while等。
We were having dinner when they came.
他们来的时候，我们正在吃饭。
I was busy yesterday. I was preparing for the exams.
昨天我很忙，我在准备考试。
过去进行时的一般形式：
肯定形式：was/were + doing;
否定形式：was/were + not + doing;
一般疑问句形式：把was或were置于句首。
I was reading aloud while my brother was watching TV.
我弟弟在看电视的时候我大声读书。
She wasn't doing her homework when I got home.
当我回到家的时候，她没有做作业。
Were you cooking the breakfast at eight yesterday morning?
昨天早上8点你在做早餐吗？
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