这个考点我把它总结为【用when引导时间状语从句的句型结构搭配】：
be about to do sth when 刚要,即将；正要做某事,突然发生其他事
be doing sth when 正在做某事突然
had done sth when 刚刚做过某事突然
be at the point of doing sth when 就在做某事的关键时刻突然
scarcely...when/hardly...when 几乎未来得及就…；刚一…就…
no sooner...than 一…就…