reason后加that,why从句时的区别是什么?

eason在定语从句中作定语时,用关系代词that/which：The reason that he gave for his absence didn't hold water.他给出的他缺席的理由站不住脚.
或在句中作主语：That was the reason that/which explains his absence.那就是说明他缺席的原因.
reason在定语从句中不作主语和宾语时,则用关系副词why有人把它叫定语从句,也有的语法学家把它叫同位语从句：That was the reason why he was absent.那就是他缺席的原因.