名詞性從句
在句子中起名詞作用的句子叫名詞性從句。名詞性從句的功能相當(dāng)于名詞詞組,它在復(fù)合句中能擔(dān)任主語(yǔ)、賓語(yǔ)、表語(yǔ)、同位語(yǔ)、介詞賓語(yǔ)等, 因此根據(jù)它在句中不同的語(yǔ)法功能,名詞從句又可分為主語(yǔ)從句、賓語(yǔ)從句、表語(yǔ)從句和同位語(yǔ)從句。

主語(yǔ)從句的用法
一. 主語(yǔ)從句在復(fù)合句中作主語(yǔ), 它可以位于句首, 但常見(jiàn)的主語(yǔ)從句多放在句末,句首則用形式主語(yǔ)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: 連詞that, whether在從句中不擔(dān)任句子成分,只起連接作用,不能省略 (注:if不可用來(lái)引導(dǎo)主語(yǔ)從句）
二. 用it 作形式主語(yǔ)的結(jié)構(gòu)
(1) It is＋名詞＋從句
It is a fact that … 事實(shí)是…
It is an honor that …非常榮幸
It is common knowledge that…是常識(shí)
(2) It is＋形容詞＋從句 (多用 should)
It is natural that… 很自然…
It is strange that… 奇怪的是…
(3) It is ＋ 不及物動(dòng)詞＋從句
It seems that… 似乎…
It happened that… 碰巧…
It appears that… 似乎…
(4) It is ＋ 過(guò)去分詞＋從句
It is reported that… 據(jù)報(bào)道…
It has been proved that… 已證實(shí)…
It is said that… 據(jù)說(shuō)…
it 作形式主語(yǔ)和it引導(dǎo)強(qiáng)調(diào)句的比較
it 作形式主語(yǔ)代替主語(yǔ)從句,主要是為了平衡句子結(jié)構(gòu), 主語(yǔ)從句的連接詞有變化。而it引導(dǎo)的強(qiáng)調(diào)句則是對(duì)句子某一部分進(jìn)行強(qiáng)調(diào),無(wú)論強(qiáng)調(diào)的是什么成分,都可用連詞that。被強(qiáng)調(diào)部分指人是也可用who/whom。例如:
判斷:
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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