题目内容：

He bought some food in the supermarket last Monday.（用at four last Monday afternoon 代替划线部分
改写句子）He _____ -______ some food in the supermarket at four last Monday afternoon)

优质解答

He was buying some food in the supermarket at four last Monday afternoon.
过去某一时刻正在做的事情,使用过去进行时.（这道题的考点）

追问：

The boy called the police while the alien was looking at the souvenirs(改为同义句） _______ the boy called the police,the alien _______ _______at　the　souvenirs．

追答：

When...........was looking.....