当先行词是reason,且在定语从句中作原因状语时,可以用关系副词why或关系代词that,也可以省略。

Question

例如：The reason (why/that) he failed was his laziness. 他失败的原因是因为他懒惰。在这句话中，why引导的是定语从句，且在从句中作原因状语。那么that 是引导定语从句还是同位语呢？如果是同位语的话，that也是不能够省略的！望各位高手指教~

Answer 1

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。why还可省去。
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作主语带有定语从句修饰时，其后的表语从句通常用that不用because。
The reason why he didn't come in time was that he didn't feel well.
他没有来的原因是因为他不舒服。
3.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。
4.注意句型This/That's why… This/That's the reason why…
That's why he did it.(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.(口语中可用that代替why)

Answer 2

这里不是that 引导的同位语从句，所谓同位语就是二者说的是同一回事，
但是 the reason 和 that he failed 不是一回事，换句话说，that 从句中看不出reason 是什么。
同位语从句的that 不能省略。

Answer 3

that引导的仍是定语从句，因为:：
当先行词是表示原因或方式的名词时，可以用that引导定语从句，相当于介词+which，that可以省略。