黑子学英语，干货分享，从什么时候开始都不晚！
今天分享托福雅思精选100句中第32句的分析，语法重点是主语从句的特征与用法。
That xenon could not form chemical compounds was once believed by scientists.科学家曾相信：氙气是不能形成化合物的。
句型结构：这是一个含有主语从句的主从复合句，主句结构为“主-谓”，That xenon could not form chemical compounds是that的主语从句；谓语为was once believed，是被动语态；by scientists是状语
在主从复合句中充当主语的从句就是主语从句。主语是一个句子要描述说明的对象，它是整个句子的主体。
主语从句是一种名词性从句。前面在分析其他句子时提到过，名词性从句是指起名词作用的从句，它包含主语从句、宾语从句、同位语从句、表语从句。
主语从句的引导词主要有四类：that, whether, 关系代词what, 连接代词（who, which），连接副词（why, when, where, how）
第一，连词that引导主语从句时，that在句中不充当成分，也没有词义。如果它放在句首，不能省略
【例】That Tom became an teacher may have been due to his mother’s influence.
【例】That Tom was chosen made a tremendous stir in his company.
在今天分析的第32个句子里，就是that引导的主语从句。
当使用形式主语it代替主语从句放在句首，而that引导的真正的主语从句放在句尾时，则可以省略that。
【例】That Tom should join the meeting is important. That放在句首不可省略
【例】It is important (that) Tom should join the meeting. It作形式主语放在句首，主语从句的引导词that可省略
第二，连词whether引导主语从句
【例】Whether it will work remains to be verified.
第三，关系代词what引导主语从句
【例】What I need is this.
【例】What Tom want to know is the truth.
第四，连词代词引导主语从句
【例】Whichever of you arrives on time will receive a gift.
第五，连词副词引导主语从句
【例】When you arrive does not matter.
【例】How this happened is no business of yours.
前面有提到用it来作句子的形式主语，将真正的主语从句放在句尾，是为了考虑句子的平衡，避免主语太长，头重脚轻。那么，什么情况下可以用形式主语呢？
第一，一般由连词that引导主语从句时，可用形式主语代替主语从句
【例】That Tom should join the meeting is important. = It is important (that) Tom should join the meeting.
本次分析的第32个例句就可以转换成It was once believed by scientists that xenon could not form chemical compounds.
第二，由连接代词或连接副词引导主语从句时，也可用形式主语放在句首
【例】Whether it will work remains to be verified. = It remains to be verified whether it will work.
第三，当句子是疑问句式时，必须要用形式主语
【例】It is true that you are Tom’s teacher?
【例】How is it that the project is fail again?
第四，关系代词what引导主语从句时，很少使用形式主语it的句式
用形式主语it结构时，常见的句型有：
第一．It + be + 名词 + that从句， 如it is a fact that……
第二．It + be + 形容词 + that从句， 如it is important that……
第三．It + 不及物动词 + that从句， 如 it seems that……
第四．It + be + 动词的过去分词 + that从句， 如 it is said that……
