1.首先大家要知道主语从句的概念、用法。要不然无从谈起：
主语从句，即在复合句中充当主语成分的句子。例如：
That the moon travels around the earth once every month is known to everyone.
众所周知，月亮每月绕地球运行一周。
此复合句主要描述的是月亮每月绕地球运行一周这个主语是众所周知的；月亮每月绕地球运行一周是该复合句的主语，因此“That the moon travels around the earth once every month”就是该复合句的主语从句。
引导主语从句的连词主要有:
从属连词:that whether if
连接代词:who whoever whom whose what whatever which whichever
连接副词:when where how why whenever wherever however
注意：
①连词位于句首时不能省略。
That she was chosen to participate in the festivals makes us very happy. (√)
她被选中去参加庆祝活动使我们很高兴。
She was chosen to participate in the festivals makes us very happy. (×)
②if和whether都可以引导主语从句；只是if引导的主语从句不能位于主句之前，且if不能和or not放在一起连用；但是if……or not结构与whether……or not结构却都是正确表达：
Whether we will go camping tomorrow depends on the weather. (√)
明天是否将去野营要看天气了。（全国高考真题）
If we will go camping tomorrow depends on the weather. (×)
Whether we will go for an outing tomorrow remains unknown. (√)
我们明天是否去郊游还不得而知。
If we will go for an outing tomorrow remains unknown. (×)
It’s still unknown whether or not we will go for an outing tomorrow. (√)
我们明天是否去郊游还不得而知。
It remains to be seen whether or not this idea can be put into practice. (√)
这一想法能否付诸实践还有待于观察。《牛津高阶英汉双解词典》
It’s still unknown if or not we will go for an outing tomorrow. (×)
It’s still uncertain if/ whether he’s coming or not. (√)
他是否来还不能肯定。
She said that she and my schoolmates all wished me success, but it didn’t matter if/ whether I would win or not. (√)
她说她和我的同学们都祝我成功，但我是否会赢并不重要。(高考真题)
2. 当主语从句过长时，为了防止句子头重脚轻，影响理解，通常把形式主语it放在句首位置，真正主语搁置于句末。
例如：That he has made a very important discovery in chemistry is true.
主语从句为“that he has made a made a very important discovery”，做主从复合句的主语， “is”为系动词，“true”为表语，此时，句子主语过长，会出现头重脚轻的现象，不利于理解，为避免这种情况，此主从复合句写成以下形式会更好。
It is true that he has made a very important discovery in chemistry.
用it做形式主语的常用句型：
(1) It is +名词+that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
It is a pity that …遗憾的是…
It's no secret that……
It's no wonder that……
It's no secret that there is a lot to put up with when waiting tables, and fortunately, much of it can be easily forgotten when you pocket the tips.
站在桌边等待顾客点菜时需要容忍很多事情，这已经不是什么秘密了。幸运的是，当你拿到小费时，就会忘记很多的不满。
It is a pity that you didn't attend Mary's wedding party.
你没参加玛丽的婚宴真是遗憾。
It's an ignorance we can ill afford.
这是我们难以承受的无知。
It is a proven fact that fluoride strengthens growing teeth.
氟化物可以强化生长中的牙齿，这是已证明的事实。
It's a sad fact that many of those killed were children.
让人难以接受的是遇难者中很多是孩子。
It's an open secret that the security service bugged telephones.
安全部门窃听电话是人人皆知的秘密。
It is a well-known fact that caffeine is a stimulant.
咖啡因是兴奋剂，这是人所共知的事实。
It is a fallacy to say that the camera never lies.
说照相机绝不骗人，这是谬见。
It is a principle that underlies all the party's policies.
这是贯穿该党各项政策的一条准则。
It is a scandal that such a large town has no orchestra.
这么大一座城市，竟然没有一支管弦乐队，真是说不过去。
It's no wonder that you are late again.
难怪你又迟到了。
It is no accident that men fill most of the top jobs in nursing.
男人担任护理中大多数最重要的工作绝非偶然。
It's no joke when your brakes fail on the motorway.
在高速公路上行驶时刹车失灵的后果是非常严重的。
It is no thanks to the Government that net assets did rise.
净资产的确增长了，但这与政府无关。
It's no business of yours who I invite to the party.
你无权过问我邀请谁参加聚会。
It is no coincidence that the works in the 'standard repertoire' tend to have names.
在“标准剧目”中的作品有各自的名字并非偶然。
(2) it is +形容词+that从句
It is natural that… 很自然……
It is strange that… 奇怪的是……
It is likely that 可能……
It is obvious that很明显……
注意！在“It is
necessary/important/surprising/strange/unthinkable/unbelievable/incredible+that从句”结构中，从句谓语常用“should——动词原形”形式，其中should可省略。
例句：
It is important that a colleague student (should) learn English well.
大学生学好英语是重要的。
It is necessary that you (should) remember these words.
你有必要记住这些词。
(3) it +不及物动词+that从句
It seems that… 似乎……
It happened that… 碰巧……
(4) it is+过去分词+that从句
It is reported that… 据报道……
It has been proved that… 已证实……
It's often said that……
It's often said that giving beats receiving.
人们常说给予胜过收受。
It is said that this medicine can alleviate pain.
据说这种药物能够缓解疼痛。
It is said that all ships coming from abroad should be performed quarantine.
据说对所有国外来的船舶都要进行检疫。
It is said that she lived to be over 100.
据说她活了100多岁。
It's said that the story happened on West Lake.
据说这个故事发生在西湖。
注意！
在“It+be+
suggested/advised/ordered/requested/required/insisted/required/demanded+that从句”结构中，that从句应用“should+动词原形”，should也可省略。
例句：It is suggested that the meeting (should) be put off.
这次会议应该推迟召开。
例句：It was ordered that we (should) arrive there in half an hour.
我们必须在半个小时内到达那里。
（5） It+特殊动词+that从句（或主语从句）
It seems to me that you don't like the idea.
在我看来，你好像不喜欢这个主意。
It really matters that you have won the game.
你们赢了这场比赛，这的确很关键。
It turns out that everything goes well.
结果是一切顺利。
It matter if/whether ……，……有关系（重要）
It will make …if/whether……，……有关系（重要）
I don’t agree with your decision, Does it matter if/whether an engineer is a man or a woman?
我不同意您的决定。工程师是男的还是女的，那有什么关系呢？
What difference will it make if he knows or not?
他知不知道有什么关系吗？牛津高阶英汉双解词典