The reason why he was late was his always being late for school.

问题描述:

The reason why he was late was his always being late for school.

The reason why he was late is that he is always late for school. 这才是正确说法.

The reason he was late for school is because he is used to being late. 我觉得这个说法最好。但是i am your baby说得也是可以~

The reason why he was late is he is always be late for school.

The reason why he was late is that he is always be late for school.

语法上是没有错的,但是逻辑上行不通.
后面的his always being ...是分词-ing形式以属格形式做表语,表现在以及主动.
中文翻译是:他之所以迟到的原因是他总是上课迟到.
感觉怪怪的.