海派小小甜心
主语从句，即在复合句中充当主语成分的句子，例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。
拓展资料
主语：是执行 句子的行为或动作的主体，如“我写字”中的“我”，就是主语，它做出“写”这个动作。“写”则是 谓语，用来修饰主语，而“字”是接受谓语“写”这个动作的对象，它因此被称为 宾语，有的语法书也称它为“ 客体”或“ 受体”。
轻舞迷影
主语从句，即在复合句中充当主语成分的句子。
That he finished writing the composition in such a short time surprised us.
他在很短的时间内写完作文让我们很惊讶。
主语：he finished writing the composition in such a short time
谓语：surprised
宾语：us
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
It作形式主语和it引导强调句的比较
主语从句常用it作形式主语，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句例如：
It is still a question whether she will come or not.
It is strange that you should like him.
It is still unknown which team will win the match.
以上内容参考：百度百科-主语从句
vincent'sir
主语从句的定义：主语从句是指在复合句中充当主语的句子，它是名词的拓展，属于主语的第三个表达层次。所以，主语从句和名词一样通常是出现在谓语动词的前面。例句1：Whether he will pass the examination is still unknown.他是否能够通过考试仍然不知道。例句2：What he said inspired me a lot.他说的话让我受到了很大的鼓舞。主语从句的引导词：主语从句的引导词与宾语从句，表语从句和同位语从句的引导词几乎相同（表语从句有几个特殊引导词除外）请参照本站文章表语从句的用法讲解。以下就按照引导词在主语从句中的意义和功能分别讲解。that引导主语从句例句1：That youth is wholly experimental is known to us all。 众所周知，青春就是探索的岁月。例句2：That she finished reading an English novel surprised us all.她读完了一部全英文小说让我们大吃一惊。Whether引导主语从句例句1：Whether she will come to the party makes no difference.她来不来参加聚会都没有关系。例句2：It makes no difference whether/if she will come to the party.她来不来参加聚会都没有关系。Who, what, which引导主语从句例句1：Who will be the leader of the team has not been decided yet. 谁会成为这个队的领头人还没定下来。例句2：What the kids need is a peaceful growing environment. 孩子们需要的是一个和平的生长环境。whose, what, which引导主语从句例句1：Whose face will flush is the liar. 谁的脸红了谁就是骗子。例句2：What goal you set now determines what your will gain in the future.你现在设立什么目标决定了你将来会有什么收获。when, where, how, why引导主语从句例句1：Why Tony did this remained a mystery. 为什么托尼做这件事仍然是个谜。例句2：When we will have our winter holidays is still uncertain.我们什么时候放寒假还不确定。Wh-ever连接词引导主语从句例句1：Whoever has a strong will is likely to become successful.凡是有坚强意志的人都有可能成功。例句2：Whatever is worth doing at all is wroth doing well.凡是值得做的事情就值得做好。
