从本质上理解，仅仅是定语从句修饰的先行词指示特别明确，不具备选择性，就只能用that, 因为that 指的就是那一个，而which指的是哪一个。
1、先行词是everything，anything等不定代词时。2、先行词被序数词或最高级修饰时。3、先行词是并列结构，既有人又有物时。4、主句的主语是疑问词who或 which时。5、先行词前有the same修饰时。
定语从句在下列情况下用that而不用which：
1，当先行词为 all, anything, everything, nothing, something, few, little, much, any 等不定代词时，如：
Do you have anything that must be claimed?
你有东西需要申报吗？
I'll do all that is the party needs me to do well.
党需要我干什么我就干好什么。
2，当先行词被 all, any, little, no, few 等词修饰时，如：
There are few rules that have no exception.
很少规则没有例外的。
All the poisons that are absorbed by the body are brought to the liver.
身体吸收的有毒物质都被送到肝脏。
3，当先行词为形容词最高级或被最高级修饰时，如：
This is the highest score that I can get.
这是我所能取得的最高分数。
This is the most interesting book that I’ve ever read. 这是我读过的最有趣的书。
4，当先行词被序数词修饰时，如：
The first thing that needs to be done is to wash your hands when you go home.
当你回家时第一件要做的事是洗手。
The second approach that we’ve taken doesn’t work. 我们采取的第二种方法不起作用。
5，当先行词为数词或被数词修饰时，如：
The two articles that I recommend to you are written by Mark Twain.
我推荐给你的两本书的作者是马克·吐温。
There are three that we found in that room.
我们在房间里发现了三个人。
6，当先行词被 the very, the only, the same, the last 等修饰时，如:
He is the very man that we need .
他正是我们需要的人。
That's really the only thing that hurts.
只有这件事真的令我伤心。
7，当句子的主语为which 时，如：
Which is the product that we desire?
哪一个是我们所期望的产品？
8，当先行词同时包含人和物时，如：
The old man and his daughter that we saw yesterday did not come here again.
昨天我们见到的那位老人和他的女儿没有再来这里。
