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.
二、物殊情况：
尽管有以上基本原则,但要学好定语从句更应注意以下特殊情况；
Ⅰ、当定语从句的先行词为表示人的不定代词,如 anyone,anybody,everyone,everybody,noone,nobody,all, these ,those等时,用who而不用that
Those who break the rule are pulished.
Anybody who had eyes could see the elephant was like a snake.
Ⅱ.以who或which作主语的特殊疑问句中先行词指人,引导词只用That,如:
a.Who is the person that is standing at the gate ?
b.Which of you that knows something about English doesn’t know this word?
Ⅲ、先行词指物时引导词用that而不用which的情况:
1、先行词有序数词或形容词最高级修饰时,
This is the best composition that has been written in English.
2、先行词是all,something,anything,nothing,everything,little,few,much等不定代词时
There is nothing/little/much that we can do for her.
3、先行词被all,every,no,some,any,little,much等修饰时
I’ve read all the books that you gave me.
4、先行词被the only/very等修饰时
This is the very/last place that they visited yesterday.
5、有两个以上先行词,分别表示人或物时
They talked about the teachers and the schools that they had visited.
6、先行词为基数时
Yesterday I caught two fish,Now you can see the two that are still alive in a basin of water.
Ⅳ.time为先行词时,前边如有序数词修饰时,定语从句引导词用that或什么也不用,如没序数词修饰时,用when或that均可
a.This is the last time(that)I shall give you a lesson.
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.