一、引导名词性从句的连接词
引导名词性从句的连接词可分为三类：
连词：that（无任何词意）whether,if（均表示“是否”表明从句内容的不确定性）as if ,as though（均表示“好像”，“似乎”）以上在从句中均不充当任何成分
连接代词：what, whatever, who, whoever, whom,whose, whichever,whomever
连接副词：when, where, how, why,how many,how much,how often
不可省略的连词：
1. 介词后的连词
2. 引导主语从句和同位语从句的连词不可省略。
That she was chosen made us very happy. We heard the news that our team had won.
比较：whether与if 均为"是否"的意思。 但在下列情况下，只可用whether：
1. whether引导主语从句并在句首2. 引导表语从句 3. whether从句作介词宾语4. 从句后有"or not"
Whether he will come is not clear.
大部分连接词引导的主语从句都可以置于句末，用 it充当形式主语。
It is not important who will go. It is still unknown which team will win the match.
二. 主语从句
1、作句子主语的从句叫主语从句。主语从句通常由从属连词that，whether，if和连接代词what，who，which，whatever，whoever以及连接副词how，when，where，why等词引导。that在句中无词义，只起连接作用；连接代词和连接副词在句中既保留自己的疑问含义、又起连接作用，在从句中充当从句的成分。例如：
What he wants to tell us is not clear. 他要跟我们说什么，还不清楚。
Who will win the match is still unknown. 谁能赢得这场比赛还不得而知。
It is known to us how he became a writer. 我们都知道他是如何成为一名作家的。
Where the English evening will be held has not yet been announced. 英语晚会将在哪里举行，还没有宣布。
有时为避免句子头重脚轻，常用形式主语it代替主语从句作形式主语放于句首，而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下：
（1） It + be + 名词 + that从句（2）It + be + 形容词 + that从句（3）It + be + 动词的过去分词 + that从句（4）It + 不及物动词 + that 从句
另注意在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时，谓语动词要用虚拟语气“(should) +do”，常用的句型有：It is necessary (important, natural, strange, etc.) that … It is suggested (requested, proposed, desired, etc.) that…
2、第一部分：常规主语从句，即句子在复合句中充当一个主语
主语从句的时态：不受主句的时态影响和限制。
（1）That he finished writing the composition in such a short time surprised us all.
（2）Whether we will go for an outing tomorrow remains unknown.
（3）Who will be our monitor hasn't been decided yet.
（4） Whom we must study for is a question of great importance.
（5）What caused the accident remains unknown.
（6）Whatever you did is right.
（7）Who the watch belongs to was lost is unknown.
（8）What we need is time.
（9）What we need are good doctors.
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