at times:
有时；不时
[例句]
The tide is,at times,very high.
潮水有时涨得很高.
I like Bob but he's very annoying at times.
我喜欢鲍勃,但是他有时也很烦人.
at a time
作“一次,每次”讲,常用于“数词+at a time”的结构中.例如：
Enter two at a time.
每次进来两人.
He checked them off one at a time as they came in.
他们进来时,他一个一个地核对.
注意它与at one time 的区别.at one time一般指“过去某一时期,曾一度(once)”或“同时”的意思.如表示“同时”,则两者可通用.例如：
At one time I used to like her,but not any more.
我曾一度很喜欢他,现在可不喜欢了.
in a time 和on a time 都是在一个时间的意思,另外还有2个短语需要也很重要in time和on time
in time
(1) Before a time limit expires.
及时：在时限到来之前
(2) Within an indefinite time; eventually:
最后,终于：在不定的时间内；最终：
In time they came to accept the harsh facts.
他们最终承认了严酷的事实
on time
(1) According to schedule; punctual or punctually.
按时,准时：按时间表的；准时的（地）
希望可以帮到你,不懂可以继续问