在托福考试备考中，语法问题一直以来都是很多同学们比较头疼的一个问题，并且托福阅读、写作以及口语等方面都对学生语法方面有着较高的要求。今天A加未来小编就为大家讲解托福语法中有关主语从句的相关内容，一起来了解一下吧！
1.主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。
好了，以上就是A加小编关于托福语法中有关主语从句的内容指导，希望能够帮助大家更好的熟悉这部分语法内容。更多托福考试问题，欢迎随时咨询我们！