同学你好，该知识点来自沪江网校《新概念英语第二册【随到随学班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学好：
定语从句中，that 可以指代人或物；who 只指代人，which 只指代物；
因此，在限制性定语从句中，多数情况下，that 都可以代替who 和which。
如：
The boy who/that helped yesterday is Tim.
The book which/that I bought last week is interesting.

但有些情况下，只能用that ——
1. 先行词是all , much, few, little, everything, anything, nothing 等不定代词时。例如：
Is there anything that you want to buy?

2. 先行词被all, few, little, much, every, some, no等词修饰或被the only, the very, the same, the last 等限定词修饰时。例如：
These are all the pictures that I have seen.
This is the very dictionary that is of great help.

3. 先行词既指人又指物时。例如：
My father and his teacher talked a lot about the things and the persons that they could remember.

4. 关系代词在定语从句中作表语时。例如：
Our school is not the one that it used to be.

5. 先行词被形容词最高级或序数词修饰时。例如：
This is the best English film that I have ever seen.
The first English novel that I read was A Tale of Two Cities.

6. 主句是who 或which 引导的特殊问句时。例如：
Who is the girl that is standing under the tree?
Which is the machine that we used last Sunday?

祝学习愉快！