at a time :一次；每次；在某时
Let your listener answer one question at a time.
让你的听众一次回答一个问题.
at one time :曾经,一度；同时(大多时候可以同at a time 替换）
All showed signs that,at one time,they had been active.
所有迹象表明,它们曾一度活跃过
at the time ：在那时,那时候
At the time,printing itself was not a new idea.
在当时,印刷术本身并不能算是一个新想法.
at all times：一直；始终
Please pay attention to them at all times.
请随时注意遵守这些规则.