定语从句中reason 后关系词的问题!先行词是reason ,后面关系词用why 还是 that 还是for which?有什么区别?

严格来讲，应该看关系代词是否在句子中充当主语或宾语，来确定关系代词that或副词why=for which。
This is the reason why he was late. ( 不缺主语或宾语)
This is the reason （that）he told me.tell sb. sth. (缺宾语可省略)

查查词典吧，可以看看朗文现代或是牛津

用why这是固定搭配．for which 也可以因为它等于WHY．THAT就不可以了,that可以用于人和物后面,作从句的宾语．～

for which,why用在形容词子句中,表示原因。
That is the reason why (for which) I’m so familiar with them.
这就是我如此熟悉他们的原因。
for which, why用在形容词子句中,表示原因。
That is the reason why he dislikes me.
这就是他不喜欢我的原因。
That is the reason for which he dislikes me.
这就是他不喜欢我的原因。
why和that都可以。见《薄冰高级英语语法》：
We don’t know the reason why he didn’t show up. 我们不知道他为什么没有来。
This is the reason that she did so. 这就是她这样做的理由。
其实，the reason后面引导定语从句的why和that也可以省略。如下面一句摘自《朗文英语语法》：
That wasn’t the reason (that) he lied to you. 那不是他对你撒谎的原因。