核心问题
请讲一讲 at a time 与 at one time 这两个介词短语的区别。
词典解答
at a time 是指“一次”（例句见 time ），at one time 则是指“一度”“从前（有个时期）”“过去某个时间”（at an unspecified time in the past）：
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At one time I lived in Japan. (RCD) 我曾一度住在日本。
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At one time most school teachers were men, but today there are more women than men.
(DAI) 以前中小学教员大都是男的,如今女的反比男的多了。