名词性从句包括宾语从句，表语从句，同位语从句，主语从句。定语从句不属于名词性从句。
名词性从句多由连词that，wh-疑问词或由what，whatever等关系代词引导。
1.主语从句：that在从句中不充当任何成分，其他引导词如what\how等基本由该词词义即可推断。如what因为是指物，即在从句中可充当主语或宾语，而how显然是方式状语，when则是时间状语，其他几种名词性从句中的引导词也是如此。故不赘述。
1)主语从句在句子中充当主语，句子的谓语动词用第三人称单数。如：
What is happening outside does not concern us.外面发生的事与我们无关。
2)主语从句前的that不能省略，仅起连接作用，有时为了保持句子平衡，that从句后置，而由it作形式主语。如：
It is estimated that a round-trip to Mars would take more than a year.据估计，飞到火星来回的时间要超过一年。
2.宾语从句在句子中充当宾语。这里要指出的是引导词whether/if，它在句中不做任何成分，只起连接的作用，故也称为连接词。其他则已在主语从句中说明。如：
I don't know where the sound came from.我不知道声音是从哪里来的。
Don't be satisfied with what you have achieved.不要满足于你已取得的成绩。
3.当从句放在系动词be，look，remain，seem等后，即构成表语从句。如：
The trouble is that I have lost his address.麻烦的是我弄丢了他的地址。
It seems that as if it is going to snow.看起来天要下雪了。
4.同位语从句
1)同位语从句是对与之同位的名词中心词作进一步解释。能接同位语从句的常见名词有：idea, fact, news, belief, hope, evidence, opinion, problem, truth, answer, proposal, theory, decision, discovery, problem, thought, certainty, likelihood, on condition that, on the ground, with the exception, in spite of the fact, on the pretence等。
The news that he has passed the examination is exciting.他通过考试的消息令人振奋。
I lent her the book on condition that she would return it before Sunday.我把书借给了她，条件是她在星期天之前还给我。
2)同位语从句与定语从句的区别在于：定语从句是对先行词加以修饰、限制，而同位语从句是为了说明中心词的具体内容；that在定语从句中充当成分，为关系代词，而在同位语从句中不作任何成分，仅起连词作用。如：
The fact that we talked about is very important.我们讨论的情况非常重要。(定语从句)
追问我就问what