He doesn t tell us the reason why he is ofen late for school

定语从句
关系副词why
　　关系副词why主要用于修饰表示原因的名词（主要是the reason）,同时它在定语从句中用作原因状语.如：
　　We don’t know the reason why he didn’t show up.我们不知道他为什么没有来.
　　She didn’t tell me the reason why she refused the offer.她没跟我讲她拒绝这项工作的原因.
　　与关系副词when和where不同,why可以换成that或省略.如：
　　That’s one of the reasons (why,that) I asked you to come.这就是我请你来的原因之一.
　　另外,与关系副词when和where可以引导非限制性定语从句不一样,why只能引导限制性定语从句,不能引导非限制性定语从句.如：
　　他失去工作的主要原因是他喝酒.
　　误：The main reason,why he lost his job,was that he drank.
　　正：The main reason why he lost his job was that he drank.