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it作形式主语的常见句型有哪些例子
1. It is/was + 被强调部分 + that/who ...
该句型是强调句型。被强调部分可以是主语，宾语，表语或状语。强调的主语如果是人，that 可以由 who 换用。如果把这种句型结构划掉后，应该是一个完整无缺的句子。这也是判断强调句型与其它从句的方法。
2. It is not until + 被强调部分 + that ...
该句型也是强调句型。主要用于强凋时间状语，译成汉语 “直到……才……”，可以说是 not ... until ... 的强调形式。
It was not until she took off her dark glasses that I realized she was a famous film star.
= Not until she took off her dark glasses did I realize she was a famous film star.
= I didn't realize she was a famous film star until she took off her dark glasses.
3. It is clear /obvious/true/possible/certain… that …
该句型中it 是形式主语，真正的主语是 that 引导的主语从句，常译为 “ 清楚（显然，真的，肯定……）”是主语从句最常见的一种结构。
It is very clear that he's round and tall like a tree.
= That he's round and tall like a tree is very clear.
4. It is important /necessary/ right/strange/natural... that ...+(should) do
该句型和上一个同属一个句型。由于主句中的形容词不同，that 后的从句中要用虚拟语气（should + 动词原形），should 可以省去。
5. It is said (reported, learned…) that …
该句型中的it 仍是形式主语，真正主语是 that 引导的主语从句。该结构常译为“ 据说（据报道，据悉……）”。
6. It is suggested ( ordered, required ... ) that ... +(should) do
该句型和上一个同属一个句型。主句中的过去分词是表示请求，建议，命令等词时，that 后的从句要用虚拟语气（should + 动词原形），should 可以省。常译为“ 据建议；有命令„„）”。
7. It is a pity ( a shame ... ) that ...
该句型中，that 后的从句一般用虚拟语气（should + 动词原形），should 可省去．表示出乎意料，常译为“竟然”。没有这种意义时，则不用虚拟语气。
It is a pity that such a thing ( should ) happen in your class.
这种事竟然发生在你们班上，真是遗憾！ It is a pity that he is ill. 他生病了，真遗憾！
8. It is time ( about time , high time ) that ...
该句型中that 后的从句应该用虚拟语气，值得注意的是① 常用过去时态表示虚拟．② 有时也用should + 动词原形，should 不能省。常译为“是（正是）„„的时侯„„”。
9. It is the first ( second … ) time that …
该句型要和上一个句型区别开来。该句型中的 that 从句不用虚拟语气，而用完成时态。至于用什么完成时态，由主句的谓语动词的时态决定。如果是一般现在时，后面从句用现在完成时态；如果是一般过去时，后面从句则用过去完成时态。该结构中 that 可以省去；it 有时用 this 替换，常译为“是第一（二）„„次„„”。
It is the first time I have been here.
= This is the first time I have been here.
10. It is „ since ...
该句型主要用作处理瞬间动词的完成时，又要和表示一段时间的时间状语连用的问题。主句中是时间作表语，其时态是现在时或完成时，since 引导的从句通常是一般过去时态，而且是瞬间动词。如果主句是一般过去时，从句则用过去完成时。
11. It is ... when ...
该句型中的 when 引导的是一个时间状语从句，主句中的 it 指时间，表语由具体的时间充当。常译为“当……的时候，是……”。
12. It be ... before ...
该句型主句中的 it 指时间, 主句中的时态常是将来一般时或过去时两种时态．主句中的表语多是long, not long , 3 days , 2 weeks 等表示时间段的词或短语。常译为“……之后……”。
13. It happens (seems, looks, appears ) that...
该句型中it是形式主语，that 引导的是主语从句，主句中的happen, seem 等词是不及物动词．
It happened ( so happened ) that he met his teacher in the street. 碰巧……
It seems that he will be back in a few days. 看来……
14. It takes sb. „ to do sth.
该句型中的不定式是真正的主语，it是形式主语，句型中的直接宾语是时间。常译为“做……要花费某人……”。
15. It is no good (use ) doing sth.
该句型中的真正主语是动名词短语，主句中的表语可以是no good , ( not any good ) , no use , ( not any use )。
16. It doesn't matter whether ( if ) ...
该句型中whether (if) 引导的从句是真正主语，该句型常译为“不论（是否）…… 没关系……。
17. It is kind ( of sb. ) to do sth.
该句型中的不定式短语是真正主语，如果不定式的逻辑主语是由 of 引起，主句中的形容词必须是能表示逻辑主语特征的形容词。常见的有: bad, brave, careless, clever, cruel, foolish, good (好心的), honest, horrible, kind, lazy, modest, naughty, nice (有教养的), polite, rude, silly, stupid, wise, wrong (错误的)等。 这个句型可以改写为：sb. is kind to do sth.
It is kind of you to say so.
= You are kind to say so.
18. It is necessary ( for sb. ) to do sth.
该句型与上一个同属一个句型。如果不定式的逻辑主语是由for 引起，主句中的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的形容词。常见的形容词有： ① important, necessary, natural
② easy, safe, common, normal, hard,difficult, dangerous, unusual, rare, impossible, pleasant 等。
在①中的形容词作表语可以用从句改写，如：
It is important for her to come to the party.
= It is important that she (should ) come to the party
19. It looks ( seems ) as if ...
该句型中it 无意义。 as if 引导一个状语从句。常译为，“看起来好象„„ ＂如果与事实不相符合，则用虚拟语气．
it做形式主语的句子
it 作形式主语主语从句有时为了考虑句子平衡，通常在主语从句处使用形式主语it，而将真正的主语从句移至句末。这分四种情况：(1) 对于以连词that引导的主语从句，通常用形式主语it替代主语从句：(2) 对于以连接代词(副词)引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句：(3) 对关系代词型what引导的主语从句，通常直接将主语从句放在句首。如：(4) 如果句子是疑问句，则必须用带形式主语it的结构：
1-1○连词that引导的主语从句，通常用形式主语it替代主语从句It is +名词+从句
It is a fact that… 事实是……；It is good news that … 是好消息；It is a question that … 是个问题；It is common knowledge that … 是常识类似的名词还有：a pity；a wonder；a good thing； no wonder； surprise等it is no surprise 毫不惊奇地例如
It is a mystery to me how it all happened.
这是怎么发生的，对我来说就是一个谜。It is common knowledge that the whale is not a fish.鲸鱼不是鱼，这是一个常识。It is no surprise that China is making progress on such fronts: the manufacture of large passenger aircraft and high-speed trains, research in yeast and aerospace.
中国在很多尖端领域，如大飞机制造，高铁、干细胞、
航天航空研究等领域取得飞速发展一点都不惊讶。
It is +形容词+从句
it的宾语从句it做形式主语
It代替从句作形式主语的常见句型
1. it 代替连词 that 引导的从句作形式主语。
1) it + be + 过去分词 + that 从句:
It's said that Tom has come back from abroad .
It was reported that dozens of children died in the accident .
可用于该句型的过去分词还有：known , thought , told , believed , hoped , announced ,expected , decided , suggested等，该句型常可以转换成含有宾语从句的复合句或者含有不定式作主补的简单句。如：
It's said that Tom has come back from abroad .
→People say that Tom has come back from abroad . →He is said to have come back from abroad .
2) It + be + 名词词组 + that 从句:
It's a pity that he can't swim for his age .
It's a wonder that you could finish such a hard task in such a short time .
可用于该句型的名词词组还有：an honour , a good thing , a fact , a surprise , good news等。
3) It + be /seem + 形容词 + that 从句:
It is likely that they will beat us tonight .今晚很可能他们会赢我们。
It doesn't seem that she will give us a hand .她帮我们的可能性似乎不太大。
可用于此句型的开容词还有；wonderful , true , good , right , wrong , (un)important ,useless , surprising , clear , possible , unusual , lucky , certain 等。
4) It + seem / appear / happen + that 从句:
It seems that he enjoys pop songs very much .看来他非常喜欢流行歌曲。
It appears that Tom might change his mind .看来汤姆会改变主意。
It happened that I went out that day .碰巧我那天外出了。
2. it 代替连接代词或连接副词引导的从句作形式主语。
1) It is known (not decided , not made clear , found out , discussed , being discussed , being considered , a question , uncertain等) + 连接代词或连接副词引导的从句。
It is still a question when we shall have our sports meet . 我们将什么时候举行运动会还是一个问题。
It is uncertain whether he can attend this conference or not . 他能否来开会还不能确定。
2) It doesn't matter(doesn't matter too much , doesn't make too much difference等) + 连接代词或连接副词引导的从句。如：
It doesn't matter whether we go together or separately . 我们一起去还是分开去都可以。
It doesn't matter too much to me what you do or where you go . 你做什么或去什么地方，对我都无关紧要
it作形式主语引导的主语从句和强调句型的区别是什么?
it引导的主语从句，形式主语就是it，真正的主语在句子的后面，通常是to do的不定式短语。
而强调句的结构 it is/was+被强调部分+that/who+句子其余部分，结构词it is/was和that/who都是没有实际意思的，只是构成句子结构的词而已。这个强调句只能强调名词和代词，也就是主语和宾语，不能强调其他句子成分。
你可以对比一下下面两句：
主语从句：It is kind of you to help me with my housework.
强调句：It is you who help me with my housework.
主语从句中it做形式主语的常见句型
常见句型有：
1. It + be + 形容词+主语从句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名词词组 + 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，
如：It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 过去分词+ 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等，
如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
it做形式主语 必须用that引导从句吗
不是必须..
it做形式主语，后面内容做句子真正主语，除了由主语从句充当之外，还可以由不定时，不定时短语，动名词以及动名词短语充当，
例如下面句子
It is very useful to learn English so well
It is the police's duty to protect the peope's life and properties
It is not so effective learning English without practising speaking English with others as often as possible
It is his duty taking good care of all his children.
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