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对英语复合句异同点的考查已成为近年来高考英语试题中的热点。比如，对定语从句与同位语从句的异同点的考查。由于这两类从句外部构成具有一些相似性，许多中学生遇到相关的语法考题，常常难于辨析作答。那么，这两大从句的区别究竟在哪里? 在回答这个问题之前，首先我们来一起看看下面的两道高考英语试题：

例1：The idea__________he was chosen attracted our attention.

a.which b.where c.that d.why

例2?：Did you know that pretty girl__________stood there just now

a.which b.who c.whose d.where

许多同学一看两道高考英语试题的答案选项，就有点迷糊。所给的四个答案选项中，其中三个选项(which、that 和where)在定语从句与同位语从句的引导词中都存在。以例1 来说，如果仅凭以先行词the idea 为物，引导词可以为which 或that，草草答题，就会得出“a.which”或“c.that”的答案。甚至，有部分同学发出感慨，这出题老师也太不负责任了，竟出现了两个正确选项。与此同时，同位语从句学得认真的一些同学，通过“a.which”与“d.why”对比，会果断的选择“d.why”。例2，许多考生心中暗暗窃喜，先行词that pretty girl 为人，且是一个具体的人，果断选择“b.who”选项。这样一来，上述两道选择题考查的都是英语从句的引导词。为什么会出现这种情况呢? 原因有二：1、部分英语老师过分强调定语从句的重要性，而对同位语从句持忽略态度。久而久之，学生也对其不够重视。2、由于定语从句与同位语从句具有一定的相似性，加上两者的引导词大部分相同，进而造成学习上的困难。遇到具体考题，学生往往难于辨别作答。下面，笔者将从定语从句与同位语从句的异同进行梳理，以便为高考学生有效掌握这两块语法知识点提供帮助。

Ⅰ.定语从句与同位语从句：形似

ⅰ.位置——一般都位于先行词(一般由名词充当)之后。

定语从句：

The boy who is reading The Old Man and the Sea is Frank.

There are servicemen from the UK who fought for and with Britain to defend democracy.

It was Winston Churchill，who was himself no stranger to those black dogs.

We can all enjoy the brighter future that I very much believe lies ahead.

同位语从句：

Most Americans held their belief that God is sovereign over their nation.

The fact that Mr.Smith was killed by his old friend last night was only a rumor.

He couldn’t figure out the reason why we were standing outside in masks.

ⅱ.引导词——都可用that，which，who，when，where，why 等词。

定语从句：

Do you know that handsome boy that/who stood there just now?

So many people who got sick like my dad and my husband never came home.

We remember the old days when most houses had an outside toilet.

But there are many people who have gotten sick，lost friends or family members.

同位语从句：

We held the belief that he was doing prescription drugs to sleep.

Alice has no idea who will be her new P.E.teacher next term.

He has no idea when Lucy will return his mobile phone.

The question why he got married to a widow is still under discussion.

ⅲ.从句语序——都为陈述句语序。

定语从句：

A dawn is the early morning when light appears in the sky.

This is a nominee who has already criticized Roberts for upholding the Affordable Care Act.

同位语从句：

This is the reason that they can’t give us an answer.

Next comes the question that we need to discuss and deal with the difficulties.

II.定语从句与同位语从句：质异

ⅰ.先行词——从词类上看，定语从句的先行词(中心词)可以是各类名词或代词，同位语从句的先行词多为抽象名词。

定语从句：

The tramp who has no job and moves from place to place is Tom’s sister.

George Mallory was an English teacher who loved climbing.

We are fighting to ensure the COVID-19 testing and tracing efforts，which were included in the Heroes Act.

同位语从句：

The fact that the earth is becoming warmer and warmer is clear.

The news that our team won the football game inspired all the fans.

The jury reached the conclusion that the men were guilty.

ⅱ.先行词与从句的关系——在同位语从句中，作为先行词的名词所表达的内容与同位语从句所表达的内容是相同的，两者之间可以划等号；而在定语从句中，因定语从句是限制或修饰先行词的，所以作为先行词的名词或代词所表达的内容与定语从句所表达的内容是不同的，两者之间不能划等号。

定语从句：

The young man whom you just talked to is an engineer of our factory.

(the young man ≠ whom you just talked to)

A pack is a piece of equipment on which you can plays records or tapes.

(a piece of equipment≠ on which you can plays records or tapes)

同位语从句：

We came to the conclusion that he was right.

(the conclusion=that he was right)

Who made the decision that the school should be closed?

(the decision=that the school should be closed)

ⅲ.从句的构成——定语从句通常由陈述句构成，而同位语从句有3 种构成：that+陈述句、一般疑问句和特殊疑问句。定语从句：

I didn’t know the dictionary that you were looking for.

解析：该句可改写为两个简单句，I didn’t know the dictionary.You were looking for the dictionary.从中我们可以看出定语从句that were looking for 原为陈述句.

I can’t undo the knot which is very tight.

解析：该句可改写为两个简单句，I can’t undo the knot.The knot is very tight.从中我们可以看出定语从句which is very tight 原为陈述句.

同位语从句：

Knowledge is the information that you have facts in your mind about something.

解析：该句可改写为两个简单句，Knowledge is the information.You have facts in your mind about something.从中我们可以看出后一句为陈述句。

There still exists doubt whether Yvonne received so many threats that she needed to take a bodyguard with her to school.

解析：该句可改写为两个简单句，There still exists doubt.Did Yvonne receive so many threats that she needed to take a bodyguard with her to school? 从中我们可以看出后一句原为一般疑问句。

There is some doubt when the general manger will come back.

解析：该句可改写为两个简单句，There is some doubt.When will the general manger come back.从中可以看出后一句原为特殊疑问句。

ⅳ.引导词的具体表现

定语从句：

Ice is a kind of water that has frozen solid.

解析：关系代词that 除了连接定语从句that has frozen solid 之外，还在该定语从句中作has frozen solid 的主语，不可以省略。

同位语从句：

And that’s among the reasons why I’ve established the Commission on Race and Ethnic Disparities to examine where inequality exists across our society and to set out a positive agenda for change.

解析：why 在从句中既不充当任何成份，又不可省略。

v.在定语从句中，关系副词when，where，why，who，whom，which一般有表示相应的时间、地点、原因、人和物的名词(先行词)；而同位语从句中，前面没有相应的名词。

定语从句：

Lanzhou is the city where we stop for lunch tomorrow.

We’re celebrating the lives of black public servants who helped make the UK what it is.

This is my wife who I owe everything to.

同位语从句：

And that’s the reason why we should choose the next president.

She raised a question where they stopped for lunch.

The question which color I preferred required consideration.

vi.在定语从句中，引导词who，whom，which，when，where，why 没有疑问意义，而在同位语从句具有疑问意义。

定语从句：

We need a new generation of social protection programmers which also cover people working in the informal economy.

我们需要新一代的社会保障方案，将非正规经济的就业人口也包括在内。

同位语从句：

The question who I should give the money to requires consideration.

把钱借给谁，这个问题需要考虑。

Mary has no idea which hand Tom writes with.

玛丽不知道汤姆用哪只手写字。