1 关系代词作介词宾语,即 介词+关系代词,指物只能用which
Last night I had a dream in which I became a Nobel Prize winner.
2 非限制定语从句中,指物只能用which, 亦可指整个前面的句子.
He reads English every day, which does good to his English study.
只用that
1 先行词为不定代词: one much everything all any ...
2 先行词被形容词最高级所修饰
3 先行词被序数词所修饰
4 先行词由only ,just ,very, right等修饰
5 在以which 开头的特殊疑问举重,避免重复用that.
6 先行词在从句中担任表语,从句的关系词用that
what 一般指代主语