at a time: 一次;每次;在某时(强调具体的某一次)
at times:有时;偶尔(说明这种事不止一次,时不时都会发生的,主要表达一种常态)
Let your listener answer one question at a time.
让你的听众(一次)回答一个问题.
The Gemini program would send two astronauts at a time into space.
双子座计划将(一次)把两名宇航员送入太空.
at times:有时;偶尔(说明这种事不止一次,不时都会发生的)
At times soils may diffuse into the mouth.
(有时)污垢甚至会渗入口内.