主语从句例子如下：
1、That he finished writing the composition in such a short time surprised us all.
他在如此短的时间内写完作文使我们十分惊讶。
2、Whom we must study for is a question of great importance.
"我们为谁而学习"是个很重要的问题。
3、Whether we will go for an outing tomorrow remains unknown.
我们明天是否会去郊游仍旧未知。
【定义】主语从句是在复合句中充当主语成分的句子。
一、引导主语从句的连词主要有:
1、从属连词:that、whether；
2、连接代词:who、whoever、whom、whose、what、whatever、which、whichever、whomever；
3、连接副词:when、where、how、why、whenever、wherever、however.
二、用it 作形式主语的结构：
1、It is +名词+that从句
（1）It is a fact that … 事实是……
（2）It is an honor that …非常荣幸
（3）It is common knowledge that …是常识
2、 it is +形容词+that从句
（1）It is natural that… 很自然……
（2）It is strange that… 奇怪的是……
3、it +不及物动词+that从句
（1）It seems that… 似乎……
（2）It happened that… 碰巧……
4、 it is+过去分词+that从句
（1）It is reported that… 据报道……
（2）It has been proved that… 已证实…