定语从句reason 为先行词 关系词怎样用 一般需根据该先行词在定从中的作用来定
：(1)在reason作先行词的定语从句中,若从句表原因,一般用why引导定语从句,why可以用for which替换.（也可以用that代替why,这种用法的that还可省去.）
如The reason why (for which) he has resigned is poor health.他辞职的理由是健康状况不佳.
(2)当reason用作定语从句的宾语时,引导词用that/which,而且that/which可省去. The reason (that/which) he gave for being late was that they were held up in a traffic jam.
他给出的迟到原因是由于他们被交通阻塞给耽误了.
(3)注意句型This/That's why… This/That's the reason why…
That's why he did it.(这是表语从句,不用that代替why)这就是他为什么做这事的原因.
That's the reason why he did it.(可用that代替why,这时that是关系副词)