(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forwhich替换.why还可省去.

　　①Thereasonwhy(forwhich)hehasresignedispoorhealth.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.

　　②Thereasonwhyhedidn'tcomeintimewasthathedidn'tfeelwell.

　　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.

　　③Thereason(that/which)hegaveforbeinglatewasthattheywereheldupinatrafficjam.

　　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'swhy…This/That'sthereasonwhy…

　　④That'swhyhedidit.(不用that代替why)这就是他为什么做这事的原因.

　　⑤That'sthereasonwhyhedidit.(口语中可用that代替why)