名词性从句(主语从句、表语从句、宾语从句、同位语从句)主语从句、表语从句、宾语从句和同位语从句在复合句中的充当的成分和其名称相同,分别作主句的主语、表语、宾语和同位语。上述这四种从句均被称为名词性从句。所有的名词性从句均不能用逗号分开,并且它们都用相同的关联词,关联词如下:① 主从连词:that (无意义), whether (是否), if (是否)(在句子中不充当任何成分)② 连接代词:who (谁), whom (谁), whose (谁的), what (什么), which (哪一个)③ 连接副词:when (什么时候), where (什么地方), how (怎样), why (为什么)主从连词只起连接作用,连接代词和连接副词除了起连接作用外,还充当从句某一个成分。另外,可以用whatever, whichever, whoever, who(m)ever等连接代词引导名词性从句,来加强语气。 (一)主语从句(subject clause)在主句中用作主语的主谓结构称之为主语从句。例如:That he will come to the discussion is certain.他来参加讨论是确定的。That the moon moves round the earth is well known to all of us.月球绕地球转动,这是我们大家都熟知的。When the meeting is to be held has not yet been decide.  会议什么时候召开还没有决定。主语从句放在句首,句子常常显得比较笨重,因此通常可以把it放在句首,作形式主语,而将主语从句放在后面。例如上面的句子可以分别改写为:It is certain that he will come to the discussion.It is well known to all of us that the moon moves round the earth.It has not yet been decide when the meeting is to be held. 下面再举一些例句:What they are after is profit.他们追求的是利润。That he is still alive is sheer luck.他还活着全靠运气。Who did the work is unknown.谁干了这工作无人知道。Whether she will join us won’t make too much difference.It won’t make too much difference whether she will join us. 她是否参与我们的活动无关紧要。Which way is more effective is still a question.It is still a question which way is more effective.   哪种办法效率更高还是个问题。 (二)表语从句(predicative clause)在主句中担当表语的主谓结构称之为表语从句。One idea is that fish is the best brain food.有一种看法认为鱼是最好的补脑食品。The problem is where we can hold our meeting.问题是我们可以在哪儿举行会议。My question is how information is stored in the long-term memory.我的问题是信息是怎样储存在长期记忆中的。That is what he really wants.那就是他真想要的东西。It seems that everything goes smoothly.似乎一切都进行得很顺利。