很多同学对定语从句和同位语从句容易混淆，因为两种从句的结构比较相近。本文教大家区分这两种从句的方法。在讲区分方法前，同学们首先要明白什么是同位语。
一个名词（或其它形式）对另一个名词进行解释或说明，这个名词（或其它形式）就是同位语。同位语与被它解释的词的格要一致，并常常紧挨在一起。
1. 名词作同位语
Mr. Wang, my child’s teacher, will be visiting us on Tuesday.（主语Mr. Wang 同位语）
王先生，我孩子的老师，星期二要来看我们。
2. 短语作同位语
I, the oldest girl in the family, always had to care for the other children.（主语I的同位语）
我，作为家里最大的女孩，老得照料家中的其他孩子。
3. 直接引语作同位语
But now the question comes to their minds, “Did she die young because she was a clone? （主语the question的同位语）
但是现在他们不得不思考这样的问题：“多利早死是因为它是一只克隆羊吗？
4. 句子作同位语
The girls were surprised at the fact that ocean ships can sail up the Great lakes.
巨大的海轮可以开到五大湖，让表姐妹俩感到吃惊。 （宾语the fact的同位语）
一、同位语从句一般跟在某些名词后面，用以说明该名词表示的具体内容。
I heard the news that our team had won．我听到了我们队获胜的消息。
①可以跟同位语从句的抽象名词通常有news，idea，fact，promise，question，doubt，thought，hope，message，suggestion，words（消息），possibility等。
I’ve come from Mr. Wang with a message that he won’t be able to see you this afternoon .
我从王先生那里来，他让我告诉你他今天下午不能来看你了。
②suggestion，advice，request，order，demand, wish等意为“建议；命令；要求”的名词后，同位语从句中的谓语动词通常用“should 动词原形”的虚拟语气结构，句中的should可以省略。
Our teacher gave us some advice how we(should)use the computer.
There was a suggestion that Brown should be dropped from the team.
有一项建议是布朗应该离队。
③同位语从句前名词的数：同位语从句前的名词通常用单数形式，并且往往带有限定词(word除外)加以修饰。
Where did you get the idea that I could not come？你在哪儿听说我不能来？
二、有时同位语从句可以不紧跟在说明的名词后面，而被别的词隔开。
The thought came to him that maybe the enemy had fled the city．
他突然想起可能敌人已经逃出城了。
several years later,word came that Napoleon himself was coming to inspect them.
几年以后，有消息传来说拿破仑要亲自视察他们
三、英语中引导同位语从句的词通常有：
连词that，whether,连接代词what，who，whom,whose
连接副词how, when, where等。（注：if, which 不能引导同位语从句。）
1. 连词that引导同位语从句（注：引导同位语从句的that不能省略）
The idea that you can do this work well without thinking is quite wrong.
你认为不动脑筋就能做好这件工作的想法是完全错误的。（作idea的同位语）
2. 连词whether引导同位语从句（注：if不能引导同位语从句）
The question whether we should call in a specialist was answered by the family doctor 我们是否请专家由家庭医生来定。
He must answer the question whether he agrees to it or not.
他必须回答他是否同意这样一个问题。
3、连接代词what, who, whom, whose引导同位语从句
I have no idea what size shoes she wears.
我不知道她穿几号的鞋。（what作定语）
The question who will take his place is still not clear.（who作主语）
4、连接副词when, where, how, why 引导同位语从句
We haven’t yet settled the question where we are going to spend our summer vacation.
到哪儿去度暑假，这个问题我们还没有决定。
l have no idea When he will be back．我不知道他什么时候回来。
My question how I shall get in touch with him has not been answered.
1.意义的不同
同位语从句是用于说明所修饰名词的具体内容的，它与被修饰词语通常可以划等号；而定语从句是限制所修饰名词的，是将所修饰的名词与其他类似的东西区别开来。
We are glad at the news that he will come. 听到他要来这个消息我们很高兴。（同位语）We are glad at the news that he told us. 听到他告诉我们的这个消息我们很高兴。（定语）
2.被修饰词语的区别
A．定语从句的先行词是名词或代词；而同位语从句的先行词只能是名词，而且仅限于idea，plan，fact，theory，promise，hope，news，doubt，truth，information，suggestion，question，belief，conclusion，situation thought, evidence, belief, fear, , theory, order,ability等少数名词。
①The boy who is playing football is my classmate． (定语从句 ）
②Those who work hard will succeed． （定语从句）
③The fact that he had not said anything surprised everybody．（同位语从句 ）
④The fact that you are talking about is important． （ 定语从句）
B．when，where，why引导的定语从句的先行词一定分别是表示时间、地点和原因的名词，而三者引导的同位语从句的先行词则肯定不是表示时间、地点和原因的名词。
例如：
①I still remember the day when I first came to Beijing．（ 定语从句）
②I have no idea when she will be back． （同位语从句 ）
3.引导词的作用不同 （区分两种从句的关键）
定语从句和同位语从句共同的引导词有四个：that，when，where，why。
下面把四个引导词分成两类说明它们在两种从句中的不同用法。
A．引导词that
引导定语从句的that叫做关系代词，它除了起连接作用，还在定语从句中充当一定成分，并且在意义上代表先行词；
引导同位语从句的that叫做从属连词，它只起连接作用（没有任何含义也不充当任何成分）。不能省略。
例如：
①The news that you told me yesterday was really exciting．
②We heard the news that our team had won．
B．引导词when，where，why
引导定语从句时，它们叫做关系副词，在从句中充当状语，可以转换成"介词＋关系代词"的形式；
引导同位语从句时，它们叫做连接副词，在从句中充当状语，但不能转换成"介词＋关系代词"的形式。
例如：
①I will never forget the day when （=on which）I joined the army．
②We have no idea when she was born．
③This is the house where(=in which) I lived two years ago．
④ This is the reason why(=for which) she will not attend the meeting．
⑤ The teacher had no idea why Jack was absent．
如果在从句和其所修饰的名词之间加一个系动词 is, 构成一个句意完整的主系表结构，那么这个从句就是同位语从句。反之，如果句意不完整，就是定语从句。
例如：
①The news that you told me yesterday was really exciting．
在The news 和that you told me yesterday 之间加is，则变为
The news is that you told me yesterday。句意不完整，故此从句为定语从句。
②We heard the news that our team had won．
此句中 the news is that our team had won．是句意完整的，所以从句是同位语从句
练一练吧1. Our team has won the game, _____ made us very happy.
2. The news ____ our team has won the game was true.
3.The news ______ he told me yesterday was true.
4. His suggestion ___the meeting be delayed was turned down.
A. which B. that C./ D. it
5. I have no idea ____ he will start.
A. when B. that C. what D./
6. I’ve come from the government with a message ____ the meeting won't be held tomorrow.
A. if B. that C. whether D. which
7. The thought ____ he might fail in the exam worried him.
A. when B. which C. what D. that
8. The order ____ the prisoner be set free arrived too late.
A. which B. whether C. that D. what
