如何辩别定语从句和名词性从句呢？

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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。名词性从句包括：主语从句、宾语从句、表语从句和同位语从句，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 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。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) The fact that he succeeded in the experiment pleased everybody.他实验成功的事实使每个人都很高兴。(同位语从句) 5. whether与if在名词性从句中的区别。 if一般只用于引导宾语从句，而whether可引导包括宾语从句在内的其他名词性从句。 1)连词whether引导的主语从句、表语从句不能用if来替换。 Whether he will attend the meeting is uncertain.他是否来参加会议并不确定。 2)宾语从句中，whether常与or not搭配，可以说whether or not，而不说if or not。 I don't care whether or not she will attend the meeting.她是否来参加会议，我不介意。 3)whether可用在介词后，或不定式前，if则不可。 She doesn't know whether to get married now or wait.她不知道是现在结婚呢还是等等。 4)在某些动词后面(如discuss)只能用whether,不用if。 We discussed whether we should go on climbing.我们讨论了是否继续攀登。