1.非限定性定语从句不能用that来引导的,这种情况下指人就只能用who。 例如:He doesn't like his little brother, who is three years younger than him.(此处只能用who) 2.先行词是人,但只能用that的有以下几种情况:先行词被the only, the very, 最高级,序数词等修饰的时候;另外若先行词是人和物的并列,也只能用that。 例如:He is the only student that passed the exam.(此处只能用that) 3.定语从句先行词的选择最简单通俗的方法就是瞻前顾后。瞻前就是要看前面先行词替代的是人还是物;顾后就是要看先行词在从句中所做成分。 同时要注意which可以做介词的宾语,而that不可以。