新东方网英语四级频道为大家整理了英语四级中常见的语法结构，一起来看看吧!
三、名词性从句：
在句子中充当主语、宾语、表语与同位语的句子分别叫做主语从句、宾语从句、表语从句和同位语从句， 这些统称为名词性从句。
(一)主语从句
在复合句中用作主语的从句叫主语从句。
A 为了强调主语从句的内容，可将从句置于句首
That you are leaving is a pity.
你要走，真遗憾。
What matters is how you live.
重要的是你如何生活。
Where he lives is not clear.
他住哪儿不清楚。
Who will stay makes no difference.
谁留下来都一样。
连词that引导的主语从句位于句首时，that不可以省，反意问句用it。
That he has lost his watch is not true, is it?
他手表丢了不是真的，是吗?
Whether the football game will be played depends on the weather.
足球比赛是否举行将视天气而定。
B 大部分主语从句都可以用it作形式主语
为了保持句子平衡，主语从句常用it作形式主语，将主语从句后置。用it作形式主语的主语从句结构有如下几种：
1、It is + 名词 + that从句
It's a pity that we can't go.
很遗憾我们不能去。
It is a fact that English is being accepted as an international language.
英语正在被人们接受为一种国际语言，这是一个事实。
It is no wonder that he is not hungry; he has been eating sweets all day.
难怪他不饿，他整天在吃糖果。
2、It is + 形容词 + that从句
It is certain that she will do well in her exam.
毫无疑问她考试成绩会很好。
It is probable18 that he told her everything.
很可能他把一切都告诉她了。
Is it true that the scientist will give us a lecture next week?
下周那位科学家将给我们作报告是真的吗?
3、It + 动词 + that从句
It seems that Alice is not coming to the party at all.
艾丽丝似乎不来参加晚会。
It happened that I was out that day.
碰巧我那天外出了。 (= I happened to be out that day.)
比 较
It is reported that there was a fire in the supermarket last night.
据报导超市昨晚失火了。(主语从句)
As is reported, there was a fire in the supermarket last night.
据报导超市昨晚失火了。(定语从句)
4、It is + 过去分词 + that从句
It is said that Mr. Green has arrived in Beijing.
据说格林先生已经到了北京。
It is reported that China has sent another man-made19 earth satellite20 into orbit21.
据报道中国又成功地发射了一颗人造地球卫星。
必背
用形式主语it引导的主语从句相当多，有些已形成固定的语法和译法。常见的有：
It is said that... 据说……
It is reported that... 据报导……
It is well known that... 众所周知……
It is announced that... 据宣布……
It is believed that... 人们相信……
It is thought that... 人们认为……
It is understood that... 自不待言……
It must be pointed out that... 必须指出……
It must be admitted that... 必须承认……
其他情况
It doesn't matter whether she will come or not.
她是否来这无关紧要。
It makes no difference where we shall have the meeting.
我们在哪里开会毫无区别。
It suddenly occurred22 to her that she had forgotten to lock the door.
她突然想到她忘记锁门了。
当that引导的主语从句出现在疑问句中时，宜用it作形式主语，而把主语从句后置。
Is it possible that they may complete the task ahead of time?
他们有可能提前完成任务吗?
Does it matter much that he won't be able to come tomorrow?
他明天来不了很要紧吗?
当主语从句出现在感叹句中时，宜用it作形式主语，而把主语从句后置。
How strange it is that the students are so quiet!
学生们这么安静真奇怪!
What a shame it is that you cannot stay for dinner!
你不能留下来吃饭多可惜呀!
连接代词what, whoever, whatever, whichever等引导的主语从句不宜用it作形式主语。
Is what he told us really true?
他所告诉我们的的确是真的吗?
Whichever of you want to come to our party will be welcome.
你们中任何想来参加我们晚会的人都欢迎。
C 使用虚拟语气的主语从句
在It is suggested, ordered, requested, insisted, etc. + that从句中，谓语部分用should + 动词原形，should常被省略。(详见第3章虚拟语气)
It is suggested that we (should) do the experiment a second time.
有人建议我们再做一次实验。
在It is important, natural, necessary, etc. + that从句中，谓语部分常用should + 动词原形，should常被省略。
It is important and necessary that we (should) keep the balance of nature.
我们要维持生态平衡，这是重要而且必不可少的。
在It is funny, strange, surprising, a pity, a shame, no wonder, etc. + that从句中，有时谓语部分用should + 动词原形，来表达说话者的感情色彩，此时should也可省略。
It is a pity that you (should) miss such a good chance.
真遗憾你竟然失去这么好的机会。
From:https://www.zgghmh.com/CET-4/20000727.asp