先行词是被定语从句所修饰的名词、代词。在定语从句中充当一定的部分。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。
例句：This is the book which I am looking for.
它是我正在寻找的书。
其中的“the book”是先行词，“which”是引导词。
例句：He is Mr. Robertson who comes from England.
他是来自英国的罗伯森先生。
其中的“Mr. Robertson”是先行词，“who”是引导词。
扩展资料：
根据定语从句与先行词间关系的紧密程度，我们将定语从句分为限定性定语从句和非限定性定语从句。
（1）限定性
大多数定语从句对所修饰词的意思加以限制，表示“...的人（或东西）”，称为限定性定语从句。
限定性定语从句，用来修饰和限定先行词，与先行词间的关系非常密切，它所修饰的先行词代表一个（些）或一类特写的人或物，从句用来说明先行词的性质、身份、特征等状况，如果去掉，则意思含混不清。
（2）非限定性：
非限定性定语从句起补充说明作用，也不会影响全句的理解，译为汉语时常常不译作定语，而根据句意翻译为相应的其他形式的从句。
对它与主句的关系不如限定性定语从句的关系紧密，只是对先行词作些附加说明，即使去掉，主句的意思依然清楚。在非限定性定语从句的前面往往有逗号隔开，如若将非限定性定语从句放在句子中间，其前后都需要用逗号隔开。
参考资料来源：百度百科-先行词
先行词是在定语从句中,被定语从句所修饰的名词。
1、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。
2、例句：He is Mr. Robertson who comes from England.
他是来自英国的罗伯森先生。
其中的“Mr. Robertson”是先行词，“who”是引导词。
扩展资料：
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。
That boy who/that broke the window is called Tom.
打破窗户的那个男孩叫汤姆。
2、先行词指物时，引导词用that或which
They planted the trees which didn’t need much water.
他们种了不需要太多水的树。
3、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.
这是我们去年住的房子。
4、先行词指时间时，引导词用When/on which或in which
I'll never forget the year when/ on which we worked on the farm.
我永远不会忘记我们在农场工作的那一年。
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.
谢谢
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