一个名词(或其它形式)对另一个名词或代词进行修饰，限定或说明，这个名词(或其它形式)就是同位语。当同位语部分是一个完整的句子时，它就是一个同位语从句。作为具有补充说明和进一步解释功能的成分及名词性从句之一，同位语及同位语从句在托福阅读中出现的频率是相当之高的。由于同位语及同位语从句表示与之同位的名词的实际内容，因此它通常会大幅增加句子长度，致使部分考生在阅读过程中出现句意理解困难的问题。因此，在平时的备考过程中，考生们应当注意将其作为一种长难句结构进行专门的练习。
一、同位语
同位语与被它限定的词的格式一致，通常紧挨在一起，有时也会出现间隔现象。
1)In the seventeenth century the organ, the clavichord, and the harpsichord became the chief instruments of the keyboard group, a supremacy they maintained until the piano supplanted them at the end of the eighteenth century.
在此句中，the organ, the clavichord, and the harpsichord became the chief instruments of the keyboard group与a supremacy呈现同位语关系：
17世纪时风琴、敲弦琴和拨弦琴成为键盘乐器类的主要成员，这种至高无上的地位一直由它们保持，直到18世纪末被钢琴所取代。
二、同位语从句
同位语从句是个完整的句子，它一般跟在某些名词后面，用以说明该名词表示的具体内容。
2)Satire jars us out of complacence into a pleasantly shocked realization that many of the values we unquestioningly accept are false.
在此句中，many of the values we unquestioningly accept are false 为同位语从句，它由that引导，解释了前面的名词realization：
讽刺作品使我们摆脱了自满情绪，并在愉快与震惊之中认识到：许多我们毫无疑问地接受的价值观是错误的。
注意：一般的“抽象”名词都可以在之后跟同位语从句，例如：reminder(提醒)， realization(了解)，skepticism(怀疑)，argument(争论)，rumor(谣言)，prospect(预期)，evidence(证据)，boast that(引以为豪的事)， claim (宣称)，words(消息)等：
3)There are numerous unsubstantiated reports that natural vitamins are superior to synthetic ones, that fertilized eggs are nutritionally superior to unfertilized eggs, that untreated grains are better than fumigated grains and the like.
在此句中，natural vitamins are superior to synthetic ones, fertilized eggs are nutritionally superior to unfertilized eggs, untreated grains are better than fumigated grains and the like 三个同位语从句均由that引导，补充说明前面的名词reports。
关于天然维生素优于人造维生素，受精蛋比未受精蛋的营养价值更高，未经熏蒸消毒处理的谷物比经过处理的好等等报道屡见不鲜，但都没有得到证实。
4)It has lived because readers appreciate a refreshing stimulus, an irreverent reminder that they lived in a world of platitudinous thinking, cheap moralizing, and foolish philosophy.
在此句中，a refreshing stimulus与an irreverent reminder呈现同位语关系，而they lived in a world of platitudinous thinking, cheap moralizing, and foolish philosophy亦为同位语从句，由that引导，补充说明了前面的名字reminder：
它之所以存在，是因为读者欣赏它使人耳目一新的刺激，它毫不客气地提醒：他们活在一个充满陈腐思想、廉价的道德说教与荒谬哲学的世界里。
练习：
1.As a result, claims that eating a diet consisting entirely of organically grown foods prevents or cures disease or provides other benefits to health have become widely publicized and form the basis for folklore.
2.The argument that humans，even in prehistoric times，had some number sense，at least to the extent of recognizing the concepts of more and less when some objects were added to or taken away from a small group, seems fair, for studies have shown that some animals possess such a sense.
3. Keen observers and quick learners, crows are astute about the intentions of other creatures, including researchers, and adept at avoiding them.