1. 当关系代词用作主语时，多用主格who。例如：
He who loses hope loses all.
失去希望的人就失去一切。(先行词为代词he, they, any, all, one等时，多用who)
I met Alice, who told me that she was learning Chinese.
我遇见艾丽斯，她告诉我她在学汉语。(在非限制性定语从句中，用who)
2. 当关系代词用作介词后的宾语时，用宾格whom，不用that。例如：
The man to whom our headmaster talked just now is our English teacher.
我们校长刚才与他谈话的那个人是我们的英语老师。(介词与关系代词紧密相连时，只能用宾格whom，不可用主格who)
注意：介词与关系代词不是紧密相连时，或者说介词放在句子后面时，这时可以用主格who，也可用that，还可以省略关系代词。因此，上面的这句话还可以有如下四种说法：
(1) The man whom our headmaster talked to just now is our English teacher.
(2) The man who our headmaster talked to just now is our English teacher.
(3) The man that our headmaster talked to just now is our English teacher.
(4) The man our headmaster talked to just now is our English teacher.
第4句简洁、自然，所以口语中用得最多。
3. 当关系代词泛指人时，多用that。例如：
He is a man that is never afraid of failure.
他是个从不怕失败的人。(that用来泛指人)
4. 当关系代词出现在who开头的疑问句时，应用that。例如：
Who is the girl that is talking to Tom in English
用英语同汤姆交谈的那个女孩是谁 (避免重复使用who，以免造成误解或语义含混不清)
，我们将会及时处理。
