我们都知道，句子的基本构架是建立在五大基本句型之上的，基本句型中的成分例如主语、谓语、宾语、系动词、表语等对于一个句子来说是不可或缺的成分，在此基础上，各类修饰成分如定语、状语等的加入能让句子的结构不断壮大。事实上，这些句子成分除主干动词外，根据传递信息量的不同，都可以以单词、短语或句子的形式存在，理论上来讲，一个句子是可以无限延长的，各类长短不一的主干枝干成分穿插嵌套，就给我们阅读此类长句造成了很大的困难。大家常说的长难句，往往就包含了大量的各类从句。我们今天就从名词性从句的识别理解开始谈起。

　　在阅读长难句的时候，一个常见的基本思路是先删繁就简，即抛去句中所有的修饰成分统统不看，即便如此，充当句子主干成分的名词性从句也是我们不能放过的。在明确名词性从句三大类引导词that, if/whether, wh-/how的基础上，我们就来结合例句总结一下各类名词性从句常见的结构，做到快速识别、正确理解。

　　1.语从句常规会出现在句首，位于主句谓语之前：

　　Why and how these people spread out into central and southern Africa remains a mystery, but archaeologists believe that their iron weapons allowed them to conquer their hunting-gathering opponents, who still used stone implements.

　　2.主语从句在句首往往会显得句子头重脚轻，所以实际阅读中我们会见到大量形式主语it代替主语从句位于句首的情况，为的是达到句子结构的平衡：

　　To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of atoms.

　　It was no accident that many leading Roman soldiers and statesmen were writers of high caliber.

　　Soon it was realized that this principle of faunal (animal) succession was valid not only in England or France but virtually everywhere.

　　由以上的句子我们可以总结出三个常见的句式分别是：

　　It is + adj. + 主语从句

　　It is + n. + 主语从句

　　It is + done. + 主语从句

　　注意前两种句式中的be动词本质上是系动词，所以也可被换成其他系动词，例如：

　　Contrary to the arguments of some that much of the pacific was settled by Polynesians accidentally marooned after being lost and adrift, it seems reasonable that this feat was accomplished by deliberate colonization expeditions that set out fully stocked with food and domesticated plants and animals.

　　But as more and more accumulations of strata were cataloged in more and more places, it became clear that the sequences of rocks sometimes differed from region to region and that no rock type was ever going to become a reliable time marker throughout the world.

　　另外需要注意的是，形式主语it结构后边所跟的主语从句个数不限，可以是一个或过个并列，从以上例句就能看到这一点。

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