It was late in the evening when the accident happened at the corner of the street.

问题描述:

It was late in the evening when the accident happened at the corner of the street.
此句的it是形式主语么?还是说此处的it指的是“天”是主句的主语呢?

不是形式主语.正如你所说,“天”才是主句的主语,就像我们常说的It is early.或者It is too late.一样.
It做形式主语时,后面应该是不定式to do sth 或者主语从句that .