名词性从句
在句子中起名词作用的句子叫名词性从句。名词性从句的功能相当于名词词组,它在复合句中能担任主语、宾语、表语、同位语、介词宾语等, 因此根据它在句中不同的语法功能,名词从句又可分为主语从句、宾语从句、表语从句和同位语从句。

主语从句的用法
一. 主语从句在复合句中作主语, 它可以位于句首, 但常见的主语从句多放在句末,句首则用形式主语it。
1.That the earth is round is true.
= It is true that the earth is round.
2. Whether he will come hasn’t been decided.
= It hasn’t been decided whether he will come.
注意: 连词that, whether在从句中不担任句子成分,只起连接作用,不能省略 (注:if不可用来引导主语从句）
二. 用it 作形式主语的结构
(1) It is＋名词＋从句
It is a fact that … 事实是…
It is an honor that …非常荣幸
It is common knowledge that…是常识
(2) It is＋形容词＋从句 (多用 should)
It is natural that… 很自然…
It is strange that… 奇怪的是…
(3) It is ＋ 不及物动词＋从句
It seems that… 似乎…
It happened that… 碰巧…
It appears that… 似乎…
(4) It is ＋ 过去分词＋从句
It is reported that… 据报道…
It has been proved that… 已证实…
It is said that… 据说…
it 作形式主语和it引导强调句的比较
it 作形式主语代替主语从句,主要是为了平衡句子结构, 主语从句的连接词有变化。而it引导的强调句则是对句子某一部分进行强调,无论强调的是什么成分,都可用连词that。被强调部分指人是也可用who/whom。例如:
判断:
It is a pity that you didn’t go to see the film.
It doesn’t interest me whether you succeed or not.
It is in the morning that the murder took place.
It is John that broke the window
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