英语定语从句中，如何判断先行词，缺少什么成分，有什么方法吗

有方法。一般在关系词前的是先行词，如果分不清的话可以参考从句中的谓语动词加以判断。比如：She is a girl that/whom everybody want to talk with.(girl是先行词都是名词，that/whom关系词，what不做关系词，th...th作定语从句。从句中everybody主语，want to talk with谓语,没有宾语，那么先行词在从句中作宾语)定语从句是形容词性从句。

定语从句一般紧跟在它所修饰的先行词后面，这就可以直接找出先行词。 看你判断对错的最简单也是最直接的办法 就是 把连接词替换陈先行词放到句子中，看语义是否通顺。

先看先行词是人或物~

英语定语从句在复合句中修饰名词或代词的从句叫做定语从句。被定语从句所修饰的词叫做“先行词”。定语从句通常跟在先行词之后，由“关系词”引出。
Eg: The girl who is dancing is my sister.
先行词 定语从句（who为关系代词）
根据定语从句与先行词的关系，定语从句可以分为限制性定语从句和非限制性定语从句两种。
1、限制性定语从句是主句不可缺少的一部分。如果省去，剩下的部分（即主句）的意思就会变得不完整、不清楚。主句和从句不用逗号分开。
Eg: I have found a man who can help you.
2、非限制性定语从句只起补充说明的作用，即使省去，剩下的部分（即主句）的意思仍然完整、清楚。主句和从句间往往用逗号分开。
注意：非限制性定语从句不能用that来引导；引导非限制性定语从句的关系代词也不能省略。
Eg: She has a sister ,who is a teacher.
关系词可分为：
1、关系代词：
（1）当先行词为人时：that(主/宾)；who(主/宾)；whom(宾）；whose(定)
（2）当先行词为物时：that(主/宾）；which(主/宾)；whose(定)
2、关系副词：
（1）when: 先行词为时间名词 在从句中作时间状语
（2）where: 先行词位地点名词 在从句中作地点状语
（3）why: 先行词为the reason 在从句中作原因状语
关系代词可以代替先行词（是人或物的名词或代词），并在从句中弃主语、宾语、定语等作用。
1、关系代词在从句中充当主语或定语时不能省略；充当宾语时，则可以省略。
Eg: That’s the girl who/that teaches me.
That’s the girl (who/whom/that) I teach.
That’s the girl whose father is a teacher.
This is the house (which/that) I want.
This is the house whose window broke last night.
2、whose可以看成是其他关系代词的所有格；相当于of which/whom。
Eg: That is the book whose cover is blue.
＝That is the book the cover of which is blue.
3、who和that 作关系代词时，还可以在从句中充当表语，并且可以省略。
Eg: She is no longer the girl (who/that)she was in her childhood.
China isn’t the country (that) it used to be.
4、定语从句中宜用that的情况
（1）先行词是不定代词all, much, something, everything,anything, nothing, none, the one等。
Eg: We should do all that is useful to the people.
There is nothing that I want to say to you.
(2)先行词被every, each, any, few, little, no, some, all, just, the only, the very, the last等词修饰。
Eg: I have finished all the homework that my teacher asked us to do.
He is the very man that the police is looking for.(它就是警察所要找的那个人。)
(3)先行词是序数词或被序数词修饰时。
Eg: When we talked about Wuxi, the first that comes into mind is Tai Lake.
This is the third film that has been shown in our school this term.
(4)先行词是最高级或被最高级修饰时。
Eg:This is the best that can be done now.
The most important thing that should be done now is to tell him the truth.
(5)先行词是数词时。
Eg: I caught two fish yesterday. Now you can see the two that are swimming in the bowl.
(6)当并列的两个先行词分别表示人和物时。
Eg: The writer and his novel that you have just talked about is really well known.
(7)在一个复合句中，如果有两个定语从句，其中一个关系代词已用which和who, 为避免重复，另一个则用that。
Eg: Edison built up a factory which produced things that had never been seen before.
(8)在以who或which开头的特殊疑问句中，为避免重复，应用that。
Eg: Who is the boy that is playing basketball?
Which of the pens that we bought is yours?
(9)当主句是There be结构，先行词为物时，关系代词用that。
Eg: There is a bird that is still alive.
注意：当先行词为人时，则用who。
Eg: There is a woman who wants to see you.
(10)当关系代词在定语从句中充当标语，而先行词为物时，应用that。
Eg: My hometown is no longer the place that is used to be.
注意：
但当先行词为人时，则也可用who。
Eg: He isn’t the boy who/that he used to be.
但在先行词为物的非限制性定语从句中，由于不能由that来引导，此时可用which。
Eg:She was fond of dancing ,which her brother never was.
5、定语从句中宜用which而不宜用that的情况（先行词为物时）
(1) 当关系代词前面有介词时（即由“介词+关系代词”引导定语从句时）。
Eg: Is this the room in which Mary lives?
(2) 在非限制性定语从句中。
Eg: Bob’s dog ,which was very old,became ill and died.
(3) 在一个复合句中，如果有两个定语从句，其中一个关系代词已用that，为避免重复，另一个则用which。
Eg: Let me show you the novel that I borrowed from the library which was newly open to us.
(4) 当关系代词后面带有插入语时。
Eg: Here is the English grammar which ,as I have told you ,will help you a lot.
6、定语从句中宜用who或whom而不宜用that的情况（先行词为人时）
(1) 当关系代词前面有介词时（即由“介词+关系代词”引导定语从句时），只能用whom，不能用that,也不能用who。
Eg: The man about whom you just talked is John.
(2) 在非限制性定语从句中，只能用who或whom,不能用that。
Eg: My brother ,who/whom you know ,is tallest in his class.
(3) 在一个复合从句中，如果有两个定语从句，其中一个关系词已用that,为避免重复，另一个则用who或whom。
Eg: The students that was praised at yesterday’s meeting is our monitor who is very hard-working.
(4) 当先行词为one, ones, anyone或those时。
Eg: Anyone who was late must give his reason.
(5) 在There be结构中，以用who而不宜用that。
Eg: There’s a gentleman who wants to see you.
注意：但当先行词为物时，则用that。
Eg: There are some books that I want to read.
(6) 当先行词后有较长的修饰语时，宜用who而不宜用that。
Eg: I met a foreigner in the park yesterday afternoon who could speak Chinese very well.
7、在带有插入语的定语从句中who和whom的选用。
试比较和分析下列两组例句：
Eg: ①John is a man who I believe is honest.
在定语从句who I believe is honest中，如果将I believe曲调，剩下的便是who is honest,在语法和句意上都能成立，因此可以判断I believe是定语从句中的插入语，而who在定语从句中充当主语，因此不能用whom。
②John is a man who/whom I believe (to be) honest.
在定语从句中who/whom I believe (to be) honest中，如果将I believe去掉，剩下的便是who/whom(to be) honest,显然不合语法，因此可以判断I believe不是定语从句中的插入语，而是从句中的主谓结构（其中“I”为主语，而“believe”为谓语）。把定语从句还原为一个简单句，则是I believe the man (to be) honest。关系代词在定语从句中代替先行词the men充当believe的宾语，(to be) honest是宾语补足语。因此既可以用who,也可以用whom。
8、当关系代词在宾语从句中充当主语时，定语从句中的谓语与先行词的主语一致。
Eg: I, who am a teacher, love my students very much.
注意:
当关系代词在定语从句中充当主语，先行词为“one of the+复数名词”时，定语从句中的谓语动词通常用复数形式。
Eg:He is one of the students who know Spanish.
当关系代词在定语从句中充当主语，先行词为“the (only) one of the+复数名词”时，定语从句中的谓语动词通常用单数形式。
Eg: He is the one of the students who knows Spanish.
This is the only one of the rooms that is free now.
9、“介词+关系代词”引导的定语从句
介词+关系代词也可用来引导定语从句。这时，当先行词为人时，则只能用whom;当先行词为物时，则只能用which。
10、一些特殊的情况
(1)which引导定语非限制性定语从句时，有时可代替整个主句。
Eg:She won the match, which made her family very happy.
(2)当先行词为the way的时候，关系代词只能有三种情况：in which,that或者省略。
Eg: I don’t like the way(in which/that) you talk to your parents.
as引导的定语从句
as引导的定语从句比较特殊，它除了引导定语从句以外，还有其本身意义，意思为“像……一样”、“正如……”。
①as引导非限制性定语从句，代替整个主句，一般用于像“as we all know, as is known to all, as it is, as is said above, as is usual”等句式中。as在非限制性定语从句中充当主语、宾语或表语，且引出的定语从句的位置较灵活，可位于句首或句末，也可插入主句中间。
Eg: As is known to all, Mark Twain is a great Amerian wtiter.(as在从句中作主语)
众所周知（正如大家都知道），马克·吐温是一位伟大的美国作家。
The earth, as we all known, is round.(as在从句中作宾语)
众所周知（正如大家都知道），地球是圆的。
注意：虽然关系代词as和which都可以代替整个主句，但
as除了引导定语从句以外，还有其本身意义（“像……一样”、“正如……”）,which本身则没有意义。
Eg: Liu Xiang won the gold medal in the Olympic Games,which made us very happy.
As it’s known to all, Liu Xiang won the gold medal in the Olympic Games.
as引导的非限制性定语从句可以位于句首，即主句之前，而which引导的定语从句只能位于主句之后。
②as用作关系代词和关系副词引导限制性定语从句，并在定语从句中充当主语、宾语、表语或状语，构成“the same…as… ,such…as… ,so…as…”等结构。
Eg: Here are such questions as are often asked by students.(as在从句中作主语)
This is the same computer as I have bought.(as在从句中作宾语)
先行词有the same修饰，可以用as,也可以用that来引导定语从句，但意义有所不同。
Eg: That’s the same watch as I have.
那块手表和我有的那块相似。（不是同一块手表）
That’s the same watch that I have.
那块手表和我有的那块一样。（是同一块手表）
关系副词引导的定语从句
引导定语从句的关系副词代替与其相应的先行词，并且在从句中分别起时间(when)、地点(where)、和原因状语(why)的作用。关系副词在意义上常常相当于“介词+which”结构。

关系词 代替功能 作用
when 时间名词 时间状语
(=at/on/in/during which)
where 地点名词 地点状语
(=at/in which)
why 原因名词 原因状语
(=for which)
如果已经确定一个从句为定语从句，如何判断要选用关系代词，还是关系副词。
虽然先行词是表示时间、地点或原因的名词，但关系词在从句中不是充当状语，而是充当主语、宾语等，就不能选用关系副词，而是选用关系代词。也就是说，选择关系词和先行词在主句中所充当的成分无关，主要看代替先行词的关系词在定语从句中充当什么成分，其次再看先行词表示的是人、物、时间、地点还是原因。
Eg: I visited the office where he once worked.
I visited the office (which/that) he once worked in.
虽然关系副词常常相当于“介词+which”结构，但是并非所有的“介词+关系代词”结构都相当于关系副词。
使用定语从句式易犯的错误
（1）在定语从句中用了多余的宾语。
Eg:「误」Some of the boys I invited them didn’t come.
「正」Some of the boys I invited didn’t come.
虽然引导定语从句的关系代词在从句中充当宾语时可以省略，但是他们已经代替先行词充当了宾语，不可以再在定语从句中加入宾语。
（2）省略了定语从句中不能省去的充当主语的关系代词。
Eg:「误」Children eat a lot of sugar often get bad teeth.
「正」Children who/that eat a lot of sugar often get bad teeth.
（3）把定语从句中谓语动词的单、复数用错。
Eg:「误」Those who has finished may leave the classroom.
「正」Those who have finished may leave the classroom.
「误」He is the one of the boys who are going to attend the meeting.
「正」He is the one of the boys who is going to attend the meeting.
（4）定语从句中选错了关系词或用了多余的介词。
Eg:「误」The house where he lives in needs repairing.
「正」The house (which/that)he lives in needs repairing.
或The house in which he lives needs repairing.
或The house where he lives needs repairing.
「误」This is the time at when he is sleeping.
「正」This is the time at when he is sleeping.
或This is the time at which he is sleeping.
（5）主句中缺表语或宾语
Eg:「误」Is this factory (that) you visited yesterday?
「正」Is this factory the one (that) you visited yesterday?
或Is this the factory (that) you visited yesterday?
「误」I have said (that) I want to say.
「正」I have said all (that) I want to say.
【注】做有关定语从句的习题时，切记：
首先要注意判断从句到底是不是定语从句。
引导词在定语从句中一定要充当句子成份。
把句子中的定语从句划出来，还原成为简单句，这样，再选用引导词或判断句子是否错误、哪里错误就显得容易多。