通常讲,先行词是人时,作定语从句主语（定语从句动词的发出者）用who引导,而在who引导的特殊疑问句中必须改为that；作宾语（定语从句动词的承受者）选who(m),that均可以、但介词后只用whom,
eg.This is the boy who often helps me with my art.
This is the boy who/whom/that I often help.
This is the boy from whom I often learn.
先行词是物时,作定语从句
作主语/宾语
用which/that 引导,而在which引导的特殊疑问句中必须把which改为that,；先行词是物时,作定语从句
作主语/宾语且
先行词中含the one,the only,the same,none,nobody,little ,few,nothing,序数词最高级时必须把which改为that,但定语从句动词为不及物动词时,介词可前置,用介词＋which,也可后置用which/that 引导定语从句.
eg.This is the factory in which I've worked for 10 years.
This is the factory which/that I've worked in for 10 years.
This is the factory where I've worked for 10 years.
先行词是地点状语时,(既不是
主语也不是宾语
）用where、介词＋which引导定语从句,
先行词是时间状语时用when、介词＋which引导定语从句!
以上讲解是我一个一个地打出了的,没有从任意网页复制!而且也希望你看了后能明白定语从句引导词的选择!