同位语是一类我们不太熟悉，又常常和定语从句弄混的概念。不过，同位语在托福阅读部分起到和前面修饰的名词一样的意义，只是为了进一步说明这个名词的概念或者作用。为了帮助大家更好地了解同位语，前程百利小编和大家介绍托福阅读长难句中同位语修饰的作用。
首先，了解一下同位语的特征
所谓同位语，就是位置相同，所代表的意思是一致的，就是用来进一步补充说明名词的成分。当我们用一个完整的句子来补充说明名词时，即构成同位语从句。
1、同位语的构成：名词+引导词+从句
例如：他决定出囯深造，这促使他更加努力的学习.
简单句： He decided to go abroad for further study. This decision encouraged him to study hard.
从句：This decision that he deckled to go abroad for further study encouraged him to study hard.
2、其中同位语中的名词叫做先行词，常用抽象名词：wish, belief; discovery; fact; information; news; proof; question; reason等。
3、同位语的引导词：that: whether; why; who;
4、从句有完整的句子銪构：主+谓+宾;
特别注意：引导词在从句里面的作用：引导词在从句里面不占成分
例1：I have a wish that I can be admitted by Yale University.
(that是同位语的引导词，引导后面的句子来修饰抽象名词wish)
例2：He made clear his desire that he (should) work hard to go to college.
(that是同位语的引导词，引导后面的句子来修饰名词desire)
其次，区分同位语从句和定语从句
1)同位语从句表示同位名词的具体内容(即同位语从句和名词是可以互相替代的，意思上是等同的);而定语从句是对先行词一名词的限制、描绘或说明。
例如：A man (who sells books in a shop) is called a bookseller.(定语从句)
He told us the truth (that he fell down from his bike this afternoon).(同位语从句)
2)引导同位语从句的that是连词，在从句中不担任任何成分，但不能省 略;
而引导定语从句的that为关系代词，在从句中担住句子的某种成分，当担任定语从句中的宾语时，that可以省略。
例如：The reason (that ) he told us is not true.〈定语从句〉
Mary suddenly got the feeling that someone was watching her.(同位语从句)
3)定语从句不能用what, how, whether引导;
而同位语从句可以用what, how, whether引导
例如：There is some doubt whether he will come.(同位语)
The mother made a promise that pleased all her children.(定语从句，that做从句的主语)
最后，同位语在长难句中的作用
In order for the structure to achieve the size and strength (necessary to meet its purpose.) architecture employs methods for support that, because they are based on physical laws, have changed little since people first discovered them - even while building materials have changed dramatically
难句结构拆分：
主句部分：architecture employs methods...
修饰部分：
插人语：because they are based on physical laws.
同位语：even while building materials have changed dramatically
这里的同位语是补充说明，翻译为：即使当建筑材料己经发生了巨大的变化时
难句类型：目的+主干+that引导的定语从句+插入语+even while引导的打断同位语。
以上就是同位语的介绍以及一些作用，希望考生能够在小编整理的同位语的相关信息中，更好地了解同位语的概念和用法，从而帮助自己准确完成长难句，解决阅读难题。
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