英语中的“先行词”是什么?

先行词:是在定语从句中,被定语从句所修饰的名词.例如:
He is a teacher who teach our English.在这里定词从句who teach our English是来修饰前面的名词teacher.所以teacher就是一个先行词.
再说下它的用法：
1、先行词指人时,引导词可用Who,Whom,及that,其中当引导词在句中作主语时用Who或that；当其充当宾语时用whom或that,在非正式宾语中也可用who,也可省略.
a That boy who/that broke the window is called Tom.
b The person to whom you just talked is Mr Li.
b 句可改为：The person(that/whom/who)you just talked to is Mr Li.
2、先行词指物时,引导词用that或which
a.They planted the trees which didn’t need much water.
b .The fish (which)we bought were not fresh.
3、先行词指时间时,引导词用When/on which或in which
a.I still remember the day when/on which I first came to Bei jing.
b.I'll never forget the year when/ on which we worked on the farm.
4、先行词指地点时,引导词用Where或in which
This is the house where/in which we lived last year.
或:This is the house that/which we lived in last year.
5、whose在定语从句中作定语时,可用来指人,也可以用来指物,如:
a.This is the boy whose mother died a month ago.
b.This is the house whose windows were broken.此句可改成：
This is the house the windoues of which were broken.
6、先行词表示原因时,引导词用why,如:
The reason why he was late for school was that he had been knocked down by a bike.

被定语从句修饰的名词或者代词叫先行词，因为它几乎总是在从句的前面。

定语从句所修饰的名词

先行词是定语从句的引导词前面的被定语从句所修饰的名词、代词，在定语从句中充当一定的成分，我们可以这样认为，之所以称它为先行词，是因为修饰它的定语从句总是放在它之后，先行词总是出现在定语从句之前，而不同于普通形容词修饰名词时，形容词位于名词之前。...

就是定语从句中定语从句前面那个词，比如说He is the man who wants to go to the USA.他就是（那个想去美国的）人。
但是如果在非限制性定语从句中的话，这个定语从句即可以修饰某个词也可以修饰前面整个句子的哦