what were you doing怎么回答
句子意思是某个过去的时段正在做什么？
回答I was +doing sth.
What did you do与What were you doing的区别
What did you do ...? 一般过去时，询问过去做过什么？例如
What did you do yesterday?
昨天你做什么了？
I did my homework yesterday.
昨天我做作业了。
What were you doing ...? 过去进行时，询问过去某一时间段或某一时间点正在做什么？例如
What were you doing at 900 last night?
昨天晚上9点你正在做什么？
I was doing my homework at 900 last night.
昨天晚上9点我正在做作业。