同位语从句指的是从句在复合句中作同位语，它是同位语的一种表现形式。这种作同位语的从句看起来似乎并不复杂，但在阅读和翻译中，如果我们缺少对它的分析，就有可能造成误解。现将这种从句分析如下：
A.同位语从句的实质
从句作同位语表示与之同位的名词(短语)的实际内容，它的作用相当于名词，对前面的名词(短语)加以补充说明或进一步解释，相当于一个表语从句，它们之间的关系是同位关系，即主表关系。如果用变通的办法，把名词(短语)和"主语+be+表语"结构联系起来，就可以看出这种同位关系。试比较：
We all know the theory that matter is made up of atoms and molecules.(=We allknow the theory, and the theory is that matter is made up of atoms and molecules.)我们大家都知道物质是由原子和分子构成的这一理论。
The suggestion that we should develop the natural resources in the region has been discussed.(=The suggestion is that we should develop the natural resources in the region, and the suggestion has been discussed.)关于我们应该开发本地区资源的建议已经讨论过了。
The problem whether natural rubber is a polymer with linear chain molecules has been solved.(=The problem is whether natural rubber is a polymer with linear chain molecules, and the problem has been solved.)天然橡胶是否是线型链状分子的聚合物，这个问题已经解决了。
I have no idea when she will be back.(=I have to idea, and the idea is when she
will be back.)我不知道她什么时候会回来。
B.引导同位语从句的连接词
引导同位语从句的连接词最常见的是that。that是一个纯引导词(pure introductory word)，在同位语从句中常可省去，尤其是在口头语中。例如：
I had the impression (that)your government was undecided about assisting us. 我的印象是，你们政府还没有决定援助我们。
"Billing and cooing! That's the way (that) you think of it! That's the way (that) you talk of it!
As though it were something you had to do.""谈情说爱!这是你的想法!这是你的说法!仿佛这是你的义务似的"。
连接词when, where, why, whether, how 等也可引导同位语从句，这些连接词除有连接主句和同位语的语法功能外，还有一定的"疑问"意义。例如：
He has solved the question why he is ill. 他已经解决了他为什么生病的问题。
Have you any idea how fast sound travels? 你知道声音传播有多快吗?
连词but that 也可引导同位语从句，人它通常出现在"否定词+doubt (deny, question)等名词"结构之后。这时，but 无否定含义，仅起强调作用，常可省去；有时也可省去that而保留but。例如：
There is no question (but) that the four modernizations will be realized in China bythe end of the century. 中国在本世纪内实现四个现代化，这是没有问题的。
There is no doubt but (that) we will succeed in designing the project.毫无疑问，我们能把这项工程设计出来。
C.同位语从句的先行词
同位语从句的先行词通常是抽象名词，常见的有：idea, situation thought, fact, evidence,belief, doubt, fear, hope, question, theory, news, order, ability等等。例如：
Wives lived in fear that their husbands might be killed. 妻子们担心丈夫会战死，
提心吊胆地过日子。
How did the cheats explain the fact that the Emperor didn't feel the new clothes in him? 皇帝觉得身上并没有穿上新衣，对这个问题骗子是怎样解释的?
Is there any reason that he chose that honest old prime minister to go and find out? 他挑选那位诚实的老宰相去了解真相，有什么缘故吗?
代词it可用作同位语从句的先行词。这个it常放在某些动词或短语动词后作宾语，同时兼作其后that同位语从句的先行词。例如：
We take it that you will act according to the final agreement. 我们认为你方是会按照最后协议行事的。
In measuring electromotive force with a voltmeter, we should see to it that the
voltmeter is placed in parallel with the electric source. 用伏特计测量电动势时，
务必使伏特计与电源并联。
I owe it to you that I am still alive. 幸亏你，我现在还活着。
有的语法学家把先行词it后的that从句看作是动词或介词的真正宾语。我们认为，这种it后的that 从句应看作it的同位语，因为它相当于"主语+be+表语"结构。试比较：
You must se to it that the children don't catch cold.(=You must see to it, and i
t is that the children don't catch cold.)你要当心，别让孩子们着凉了。
D."复合介词+the fact+that从句"结构从总体来看，这种结构属介词短语。如果分开来理解，其中的the fact 用作复合介词的宾语，同时兼作其后that从句的先行词，that从句为同位语从句。例如：
It is chiefly due to the fact that the sun gets higher in the sky in summer and
shines more directly down on that part of the earth.
这主要是由于夏天太阳在空中的位置较高，同时更加直接地照射着地球的那一部分。
We must never overlook our small weak points because of the fact that we have ma
de big achievements. 我们不可因为有了大的成绩就忽视小的缺点。
Owing to the fact that I had lost his address, I didn't know where I could find
him.由于我把他的地址遗失了，所以我不知到哪里去找他。
这种结构中的复合介词很多，常见的有：due to, by dint of, by virtue of, because of, on account of, in view of,等等。
E.同位语从句与先行词之间的分隔同位语从句一般都紧跟在与其同位的名词(即先行词)之后，但当先行词作句子的主语，其谓语较短，同位语较长时，同位语从句常与同位名词(短语)分隔。例如：
In the first half of the last century the discovery was made that a magnet could
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