定语从句中什么时候只能用who不能用that

一般情况下都可以换
但是如果是这样的句子 Who ______is talking to my parents knows him?
这里的定语从句是修饰"who"的, 就只能用that了
另外,一般引导非限定性定语从句习惯上用who.

当定语从句中缺少地点状语时，关系词只能用where，而不能使用that.
当定语从句缺少表示地点的主语/宾语而不是地点状语时，关系词使用that/which，而不使用where.

如：This is the city where I stayed last year.
定语从句I stayed last year缺少地点状语，用where = in which = in the city.
= This is the city that I stayed in last year.
定语从句I stayed in last year中，in后面缺少宾语，用that，不用where.

一般用who都可以用that代替

好像都可以吧