定语从句that which省略的情况详解：
1.关系词that which who在从句中作宾语时，常可省略
（相关链接：宾语是什么）
Did you enjoy the movie that/which we just saw？
= Did you enjoy the movie we just saw?
你喜欢我们刚才看的那部电影吗？
[that/which在从句中作saw的宾语，可省]

The man that/who you met at the station is my father.
= The man you met at the station is my father.
你在车站遇见的那位男士是我的爸爸。
[that/who在从句中作met的宾语，可省]


2.介词提前时，关系词不可省略
①which 在作介词宾语时，介词可提前。而that/who则不可。
The hotel which you stayed at is famous.
你住的那个宾馆是出名的。
=The hotel at which you stayed is famous.
The hotel at that you stayed is famous. (×)

②在介词提前的情况下，which即使是作宾语，也不可省略。
The hotel at which you stayed is famous.（√）
The hotel at you stayed is famous.（×）
[省略会让句子混乱]

定语从句that which省略的情况主要是这些。若还不太理解，建议先查看：
定语从句基础用法