当定语从句所修饰的先行词指示非常明确、不具有选择性时，应使用 that，因为 that 特指“那一个”具体对象。而 which 更倾向于指“哪一个”，who 更倾向于指“谁是谁”，两者都带有一定的选择意味。这种区别体现了 that 的限定性和 which / who 的非限定性功能。
一、只用 that（指人或指物通用）
a. 先行词是指物的不定代词
如 all, little, something, everything, nothing 等（指物时只用 that）。
All that glitters is not gold.闪闪发光的东西未必都是金子。There is nothing that can stop me.没有什么能阻止我。He did everything that he could to help us.他做了他能做的一切来帮助我们。
b. 先行词被最高级或序数词修饰the first, the best, the tallest 等。
She is the tallest girl that I have ever seen. 她是我见过的最高的女孩。
The third place that they visited was the Great Wall. 他们参观的第三个地方是长城。
This is the first time that I have visited China.这是我第一次来中国。
c. 先行词被 the only / the very / the last 修饰
This is the very book that I was looking for. 这正是我在找的那本书。She is the only person that knows the secret. 她是唯一知道这个秘密的人。
d. 主句以 Who 或 Which 开头
Who is the person that you saw yesterday? 你昨天见到的那个人是谁？Which book is the one that you recommended? 你推荐的是哪本书？
Who is the person that loves you most? 谁是最爱你的人？
e. 先行词既有人也有物
The teacher and the books that he recommended were excellent.他推荐的老师和书籍都非常出色。
二、只用 which（指物）
a. 非限制性定语从句She lives in a small town, which is famous for its beautiful scenery. 她住在一个小镇上，这个小镇以其美丽的风景而闻名。
He passed the exam, which made his parents very happy.他通过了考试，这让他的父母非常高兴。（先行词为整个主句）
b. 介词+ which This is the book from which I learned a lot. 这是我从中学到很多的一本书。The room in which we held the meeting was quite large.我们开会的那个房间相当大。
三、只用 who（指人）
a. 非限制性定语从句（有逗号）
My father, who is a doctor, works hard.
b. 先行词是 people, those, one, ones 等泛指“人们”
Those who listened to him were deeply moved. 听他讲话的人都被深深地感动了。
Anyone who breaks the rules will be punished. 任何违反规则的人都会受到惩罚。
注：指人的不定代词（如 someone, anyone, those 等），who 和 that 皆可，who 更自然常见，我们考试时就用who。
三个硬规则
