一、 who引导的定语从句的'先行词是表示人的名词或代词。如：The student who is answering the question is John. 正在回答问题的那个学生是约翰。

　Anybody who breaks the laws will be punished. 任何违反法律的人都将受到惩罚。

　二、 who是主格，在定语从句中作主语，此时不能省略。如：The person who was here yesterday is a musician. 昨天来这儿的那个人是位音乐家。

　但在非正式英语中，who亦可作宾语，且可以省略。如：The man（who）I saw just now is Mr Li. 我刚才见到的那个人是李先生。

　三、 在定语从句中，who在人称、数上和其前面的先行词保持一致。如：Do you know the boy who is standing over there? 你认识站在那边的那个男孩吗？

　四、 若先行词中既有人又有物，关系代词用that而不用who。如：He watched the children and boxes that filled the car. 他看见了塞满汽车的孩子和箱子。

　五、 若先行词前面有形容词最高级、序数词修饰，关系代词用that而不用who。如：Yao Ming is the best basketball player that I know. 姚明是我知道的最好的篮球运动员。

　Tom is the first boy that left the room. 汤姆是第一个离开这个房间的男孩。

　拓展内容：定语从句中who和whom的区别

　简单地说就是他们所充当的句子成分是不一样的楼层:2

　who 可以做主语,又可以做宾语

　whom只能做宾语

　who和whom在定语从句中作关系代词时：

　1.who指人,在从句中做主语

　(1) The boys who are playing football are from Class One.

　(2) Yesterday I helped an old man who lost his way.

　2.whom指人,在定语从句中充当宾语,常可省略.

　(1) Mr.Liu is the person (whom) you talked about on the bus.

　(2) Mr.Ling is just the boy whom I want to see.