Nobody knows the reason why he is often late for school.把why改成that/which引导的定语从句
问题描述：
Nobody knows the reason why he is often late for school.把why改成that/which引导的定语从句
答
Nobody knows the reason 【for which】 he is often late for school
或许你可以这么考虑：for是“为”,which是“什么”,组合就同等替代了why“为什么” .