定语从句中什么时候先行词用that不用which？

首先，定语从句 是限制性定语从句，然后再看 先行词

当 先行词是 all，anything，none，few等 时，连词只能用that
如 Is there anything else that he has left in the room?

当先行词被all,any,no,few等修饰时 ，定语从句的连词只能用that （原理和上面类似）

当先行词被 the only，the last，the very等 修饰时，连词只能用that
如 It was the last train that we were able to take. (it这里做代词，指代鞋子，可以换成this)

当先行词是 最高级或被最高级的形容词修饰时，连词只能用that
如 This is the most beautiful dress that I have ever seen.(that常会被省略)

当 先行词 是 序数词或被序数词修饰时，连词只能用that

如 There are two dogs that bark and fight at the corner of the town every day.

当先行词为并列结构，有人有物，连词只能用that
如 He was glad to talk about the writer and his novels that had impressed him a lot.

一般来说，限定性定语从句中，先行词能用which多数可以用that替换，而可以用that未必能用which