when 后面本来有主语和 be 动词，还有动词 ing，有时be动词和动词ing可以省略，这样就只剩下了 动词ing。

例句

When I was doing my homework last night， I heard someone knocking at the door. 从句和主句的主语是相同的一个人，这时上句可以改写成

When doing my homework last night， I heard someone knocking at the door.