关系代词在从句中作主语或宾语，代替物时，在限制性定语从句中，可以用that或which，在非限制性定语从句，只用which。

以下情况要注意：

1) 当关系代词紧跟介词，作介词宾语时，不可用that，只能用whom或which。例如：

The book from which （不用 that） I got a lot of useful information was written by a famous scientist. 我从中得到许多有用信息的那本书是一位著名科学家写的。

Who is the boy with whon （不用 that） you were talking a moment ago? 刚才和你谈话的那个男孩是谁？

但是当介词位于定语从句句末时，作为介词宾语的关系代词则仍可用that，也可省去。例如，以上两句可改为：

The book （that或which） I got a lot of useful information from was written by a famous scientist.

Who is the boy （that或which） you were talking with a moment ago?

2) 当先行词是all, everything, something, nothing等不定代词，或被 first, last, only, few, much, some, any, no以及形容词最高级等词修饰时，应该用关系代词that，不用which。例如：

I am interested in all that you have told me. 你告诉我的一切我都感兴趣。

He asked for the best book that there was on the subject. 他要有关这个内容的最好的书。

3) 当并列的两个先行词分别表示人和物时，要用关系代词that引导定语从句。例如：

We were deeply impressed by the teachers and schools that we had visited there. 我们在那儿访问过的老师和学校给我们留下了深刻的印象。