在定语从句中,关系代词who (whom) 和that都可指人和物,在一般情况下,可以互换使用,但在下列情况下值得注意：
一、用who (whom) 而不用that的情形
(1) 在非限制性定语从句中的先行词指人时,只能用who (whom).
(2) 当定语从句对指人的先行词进行隔位修饰时,只能用who (whom).
(3) 当先行词为people和those时,只能用who (whom).
(4) 当先行词为one, ones, anyone, everyone, none, all(指人) 时,只能用who (whom).
(5) 先行词指人,而关系代词在从句中作主语时,一般多用who.
(6) 在there be句型中名词的定语从句多用who (whom).
(7) 当句子中有两个指人的现行词分别带有两个定语从句时,若一个定语从句的引导词为that, 那么,另一个定语从句的引导词必定为who.
二、用that 而不用who (whom)的情形
(1) 当先行词前面有形容词的最高级或序数词修饰时,引导词只能用that.
(2) 当先行词前面有only, some, any, no, every, little, few, much, all, very形容词修饰时,引导词只能用that.
(3) 当主句是以who开头的特殊疑问句时,引导词只能用that.
在定语从句中,关系代词which和 that都可指物,一般情况下,可以互换使用.但在下列情况下值得注意：
A. 只能用that而不能用which的情形
(1) 当先行词为：all, little, few, much, none 及 some-, any-, no-, every- 与 thing 所组成的复合单词时,只能用that.
(2) 当先行词前面有only, some, any, no, every, little, few, much, all, very等形容词修饰时,只能用that.
(3) 当先行词由形容词的最高级或序数词修饰时,只能用that.
(4) 当先行词同时包括人和物时,只能用that.
(5) 当先行词在定语从句中作表语时,只能用that.如：
This is a book that I have been looking for since last week. 这就是那本自上周以来我一直在寻找的书.
(6) 当主句是以which开头的特殊疑问句时,只能用that.如：
Which is the picture that you drew yesterday? 你昨天画的那张画是哪一张?
B. 只能用which而不能用that的情形
(1) 当非限制性定语从句中的先行词指物时,只能用which.如：
The pencil-case, which I bought last week, is missing. 那个铅笔盒,我上周买的,现在不见了.
(2) 关系代词前面有介词时,只能用which.如：
The tree under which we used to take a rest has been cut down. 我们过去常在下面休息的那棵树已砍刀了.
(3) 先行词为代词that或that所修饰时,只能用which.如：
I don’t take that which is too expensive. 我不要太贵的那一个.
whose
关系代词在从句中作定语表示所属关系,先行词是人,表示“先行词这个人的…”,用whose；先行词是事物,表示“先行词这个事物的…”,用whose…或the…of which或of which…都可以.
