【问】 请讲一讲at a time与at one time这两个介词短语的区别。

【答】 at a time是指“一次”(例句见at a time/at the 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)：

1. At one time I lived in Japan. (RCD)

我曾一度住在日本。

2. 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。