今天将为大家重点介绍主语从句，先来回顾从句的体系吧。
从句由名词性从句，形容词性从句和副词性从句三大句型构成。
那么，名词从句中的四个分支句型，具体又是怎样应用的呢？现在就让我们一起走进主语从句吧！
主语从句（Subject Clause）
主语从句，就是从句做主语。主语从句主要分为三类：
that引导的主语从句
“that”引导的主语从句可以放在句首，例如：
（1）That the seas are being overfished has been known for years.
海洋被过度捕捞，许多年来已经是人尽皆知的事情了。
但更为常见的是：用it作形式主语放在句首，that主语从句放在句末。
常见的主语从句的句型有：
a. It+be动词+过去分词+that从句
例如：
（2）As a child — and as an adult as well— Bill Gates was untidy. It has been said that in order to counteract this, Mary drew up weekly clothing plans for him.
童年时期，以及即使长大后，比尔·盖茨也不修边幅。据说为了改掉他的这个毛病，玛丽为他制定了一周着装计划。
（3）It is commonly believed in United States that school is where people go to get an education.
在美国人们普遍认为，学校是人们接受教育的场所。
b．It+be动词+形容词+that从句
例如：
（4）Fortunately, the White House is starting to pay attention. But it’s obvious that a majority of the president’s advisers still don’t take global warming seriously.
幸好白宫已经注意到全球变暖的问题。但是很显然，总统周围大多数顾问仍然没有认真对待该问题。
（5）It is necessary that we should master a foreign language.
我们很有必要掌握一门外语。
c. It +be动词+名词短语+that从句
例如：
（6）It is an established fact that factory workers work better, harder, and have fewer accidents when their machines are painted orange rather than black or grey.
事实证明，如果把工厂的机器漆成橘黄色而不是黑色或灰色，工人就会干得更勤奋、更出色，事故也会更少。
（7）It is a truth universally acknowledged that a single man in possession of a good fortune must be in want of a wife.
一个众所周知的事实是，一个单身汉若是非常有钱，那么他一定想要找个妻子。
d. It+不及物动词+that从句
通篇浏览下来，你会发现“it作形式主语放在句首，that主语从句放在句末”这样的句式还是很丰富地，他们的用法你能区别开吗? 做个练习试试看吧！
1. _____ hard water does not mix well with soap.
A. That is a well-known fact B. That is a well-known fact in which
C. It is a well-known fact that D. It is a well-known that
答案：C
解析：本题考查主语从句“It is+名词+that”
译文：众所周知，硬水和肥皂不能较好地融合。
2. _____ I have made no arrangements to deal with such cases.
A． It appear that B. That appears C. They appear that D. It appears that
答案：D
解析：本题考查主语从句。
译文：似乎我并没有被安排处理这类案件。
