I saw my teacher when I_(walk)in the street填什么?为什么?

问题描述:

I saw my teacher when I_(walk)in the street填什么?为什么?

was walking
叙述故事发生的背景,一般使用过去进行时。
在这句中,两件事都是发生在过去,See teacher 是主要事件,walk in the street 是背景。

was walking
过去进行时,因为saw

was walking
过去进行时,因为saw

was walking
过去进行时