The reason (why/that) he failed was his laziness. 如果是why的话定从

一、the reason why肯定是定语从句
二、the reason that 不一定都是定语从句,分以下两种情况
1.当that后面加的句子是完整的句子的话,那它肯定是同位语从句,例如
He gave me the reason that he got up too late this morning.
2.当that 后面加的句子是不完整的句子的话（也就是缺少主语、宾语或表语时）,那它就是定语从句了,例如
I believe the reason that he gave.
He gave me the reason that was unbelievable.(他给了我一个令人难以置信的解释)
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再问： The reason that he failed was his laziness. 这句话应该不对 the reason for which he failed was his laziness 如果是that的话，感觉说不通
再答： 你说的对，定语从句中，the reason做先行词时，引导词更多的用why，或用for which，而that多出现在同位语从句中，The reason that he failed was his laziness.这是同位语从句，根据同位语从句的含义，应该是对前面的词进行补充说明，而本句是起修饰限定的作用，所以应该是定语从句，而从句部分并不缺主语、宾语或表语，所以只能选择关系副词why（=for which），做状语。