when
it was+ (时刻,或时间点）+ when sth.happened.当某事发生时,是几点钟或哪一年如：
It was 1997 when Hongkong returned to China.
It was five o'clock when we arrived at the small mountain village.
对比强调句型：It was at five o'clock that we arrived at the small mountain village.
注意 第二句 有 at
It+ is/ was+…+that…多用于强调.