at one time 意思是“过去有段时间；曾经”；at a time意为“一次；有一次；在某个时候”，当其意思为“一次”时，可用at once或at the same time短语替换。例如：[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ K2JVWOK2 t5(bS~PFroG]

    At one time, Einstein helped the little girl do her arithmetic and she brought him cakes. 有段时间，爱因斯坦帮助那个小女孩做算术作业，小女孩给他带来小甜饼。[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ XEYmutBYwFV'LOFroG]

    Don't all speak at once! One at a time, please. 不要大家一起讲话。请一次一个人来讲。[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ EYUx j\IvzDAMT2D5FroG]