I dont know the reason why he was late 为什么不用that

题目

I dont know the reason why he was late 为什么不用that

答案

I don't know the reason why he was late.
我不清楚他为迟到的原因.
解析：why he was late 为定语从句 ,修饰限制前面的中心语 reason
why 是定语从句里的原因状语