83　at a time和at one time

① at a time表示“一次，再次”。

She reviewed three lessons at a time. 她一次复习了三课。

He went up the stairs two at a time. 他一次迈两个台阶。

② at one time表示“曾经，有一段时间”，常同过去时连用。

At one time I worked in a middle school. 有一段时间我在一所中学工作。

Her mother was at one time a nurse. 她母亲有一段时间当护士。