1、首先要明确的是,this/ that/ these,3个永远不能做定语从句的“先行词”.
2、that, 可以作“替代词”,带后置的定语短语（但不是定语从句）：
The weather in East China is wetter than that (the weather) in West China.
3. 只有 those (people/ things) + 定语从句：
those (poeple) + who-/whom-/that-
those (things) + that-/which-