why在定语从句中做原因状语时,可不可一换成that?

有reason(原因)这个词，用that 和 why均可，
没有这个词，得用why
the reason why （或者that）he left her 他离开她的原因 * That is (the reason) why I came early. 这就是我来早了的原因.
that is why i came early,这个就不行了，因为没了reason(原因)这个词!
楼上写的让人看不懂，楼主是问的是是否能换that ，你写个for which 做什么呀？
其实for which和that都是可以的。

why在定语从句中做原因状语时,不能换成that
请看下面我给你分析的两个句子,想必看完之后,你就会对why引导的原因状语从句的应用有一个彻底的掌握的
句①He wanted to know the reason.
句②I was late for the reason.
把句②变为句①的定语从句,句①中的the reasons做定语从句的先行词.因为the reason做介词for的宾语,先行词是the reason指物,在定语从句中介词for提前,用关系代词which替代the reason,the reason就不能在定语从句中出现了.由此得到句③He wanted to know the reason for which I was late.
注意：在定语从句中当先行词为the reason(s),定语从句由for which引导时,可以用why代替for which.由此我们得到句⑤He wanted to know the reason why I was late.
请把下面两句话合并为一句,把第二句话变为第一句的定语从句.
The reason is not very convincing.
He came for the reason.
→The reason why he came is not very convincing.
The reason for which he came is not very convincing.