that 和 which 是两个定语从句常见的引导词。关系代词that在定语从句中可以指代人，也可以指代物，而which 一般用于指物。所以当先行词为人时，多数情况下用that，如:
She is the girl that lives next door. 她就是住在隔壁的那个女孩。
[注] 当定语从句的先行词是婴儿时，常用which代表人，如:
These are the babies which need special care. 这些是需要特别照顾的婴儿。
that 和 which 在定语从句中指代物时，多数情况下是可以互换的。但下列情况下宜用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 required of me. 要我干什么我就干什么。
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。
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 moving away soil. 第一件要做的事是移除泥土。
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 for the vacancy. 他正是我们需要补这个空缺的人。
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 dog that we saw yesterday did not appear again. 昨天我们见到的那位老人和他的狗没有再出现。