My mother ___（cook）when I got home yesterday afternoon

My mother ___（cook）when I got home yesterday afternoon 说下为什么 谢谢

was cooking

根据语境，就是说，我回家的时候，妈妈正在做饭。。

were cooking

因为是昨天所以用过去时

sb + be+ doing sth when ···这是一个固定句型

My mother was cooking when I got home yesterday afternoon .

到家的时候，妈妈正在做饭。因此做饭的动作应该用过去进行时。