通过引导定语从句的词所代替的部分在定语从句中所做的句子成份来确定。
关系代词（Relative Pronoun）可以代替先行词（即：被定语从句修饰的词），是指人或物的名词和代词，并在从句中起主语、宾语和定语等作用。（各个关系代词的代替功能以及它们在从句中所起的作用见下表。）例如：
Where is the girl who （或 that） was injured in the accident?
在事故中受伤的那个女孩在哪儿？（关系代词作主语）
The man whom （或 that） you saw just now is our manager.
你刚才见到的那个人是我们的经理。（关系代词作宾语）
He found the watch which （或 that） he had lost.
他找到了他丢掉的那块表。（关系代词作宾语）
The house which （或 that） was damaged in the fire will be rebuilt soon.
那座在大火中烧坏的房子不久将被修复。（关系代词作主语）
Is there anyone here whose name is Wang Lin?
这儿有个名字叫王林的人吗？（关系代词作定语）
The building whose roof we can see from here is a supermarket.
我们在这儿看得见屋顶的那座大楼是一家超级市场。（关系代词作定语）
引导定语从句的关系副词（Relative Adverb）代替与其相应的先行词，并且在从句中分别起时间、地点和原因状语作用。关系副词在意义上常常相当于一个“介词+which”结构。
He came at a time when (at which) we were badly in need of help. 他在我们急需人帮忙的时候来到了。
The school where (in which) I work is not far from here. 我工作的那个学校离这儿不远。
Is this the place where (at which) that traffic accident occurred? 这就是那个交通事故发生的地方吗？
He didn’t give the reason why (for which) he was absent yesterday. 他没有提到他昨天缺席的理由。
The reason why (for which) he missed the bus was that he got up late. 他没有赶上车的原因是他起床晚了。
We admired him for the way (in which/that/how) he faces his difficulties. （在现代英语中how较少用）我们佩服他克服困难的方法。