(1) 在这类句型中，不定式短语，动名词短语，或that从句作主语也是可以的，但不太自然。一般是用it作形式主语，而把真正的主语(不定式、动名词短语或that从句)后置。

• It's nice to be with you again.
  又能与你在一起，太好了。
• It's difficult for me to grasp what she's talking about.
  我很难理解她在说什么。
• It's interesting that you should like a man like him.
  真有意思，你居然会喜欢上他那样一个人。
• It worried me a bit that she didn't phone.
  她没打电话，我有点不安。

 (2) 这类句子可以表示许多不同的意思，最常见的有：

①表示重要性。

• It's important that we get everything ready by May 1.
  我们五一节前把一切准备好是非常重要的。
• It's essential to book in advance.
  事先订购是很重要的。

②表示难易。

• It's hard work keeping the grass green at this time of year.
  在一年这个时候保持草坪青绿是不容易的。
• It's not easy to get people to change their minds.
  要人们改变主意是不容易的。
• It must be hard to live on your small salary.
  靠你微薄的薪水生活一定是困难的。

③表示可能。

• It's probable that we'll set off tomorrow.
  有可能我们明天就出发。
• Is it possible to go there by road?
  从陆路去那儿可以吗？

 (3) 表示有用。

• It's not very useful to read such books.
  读这种书不是很有用。
• It's pointless to go there on Monday. There's a public holiday.
  星期一去那里没有用，那是公共假期。
• It's pure waste of breath/no use/no help/no good/useless/senseless arguing with him over the matter any further.
  为这事跟他再争论下去是白费力气/没有用/没有帮助。

 (4) 表示常态或频率。

• It's unusual to see Peter with a girl.
  看见彼得与一个姑娘在一起是不常有的事。
• Is it customary to tip hairdressers in your country?
  在你们国家有给理发师小费的习惯吗？

 (5) 表示所花时间。

• It took them a whole year to get the project completed.
  他们花了整整一年才使这项工程竣工。

 (6) 表示情绪上的反应。

• It was a nuisance having to wait for such a long time.
  得等这么长时间真是烦人。
• It was really astonishing that she refused to talk to you.
  她不跟你讲话确实令人吃惊。
• It shocked me that Peter didn't tell anybody where he was.
  彼得没告诉任何人他在哪儿，这令我震惊。
• It's interesting to see different cultures and ways of life.
  看看不同文化和生活方式是很有意思的。
• It would be a pity to miss such a chance.
  失去这样一个机会将是一件令人遗憾的事。

 (7) 表示真实性。

• Is it true that Tom and Mary have got married?
  汤姆和玛丽已经结婚了，这是真的么？

 (8) 表示方便。

• When would it be convenient for you to come to my house?
  你什么时候方便到我家来？
• It's a great convenience to live near a tram or bus stop.
  住在电车或公共汽车站附近是一个极大的便利。
• Will it suit you to come tomorrow?
  你明天方便来么？
• It will suit me best for you to come the day after tomorrow.
  你后天来对我来说最为方便。

 (9) 表示一个人的想法。

• It occurred to me that she might have forgotten the date.
  我忽然想到她可能把日期忘掉了。
• It didn't occur to you to phone, I suppose?
  你就没有想到打个电话?
• It struck me that the boss was behaving pretty strangely today.
  这使我感到老板今天的举止十分奇怪。

(10) 表示由外观上得出的判断或看法。

• It appears that Geoffrey might change his mind on the matter.
  看起来,杰弗里可能会在这件事上改变主意。
• It seems obvious that we can't go on like this.
  显而易见，我们不能这样下去了。

＊在上述句型的that从句中，有些需用should,而在美式英语中，常用虚拟语气(由should +动词原形构成，或省掉should，直接用动词原形)，表示判断或评论。

• It's interesting that you should think like that.
  你居然会那样想，真是有趣。
• It's important that the people (should) be told all the facts about the situation.
  应告诉人民关于形势的全部情况，这是非常重要的。
• It's natural that he should say so.
  他这样说是自然的。

(11) 这种句型可以表示“必要，必然，适当，当然”，常用的形容词为proper, right, important, necessary, imperative, natural, good, fit, useless等。此外还可以表示惊奇、遗憾、奇异、怀疑等感情。这时，should 常译作“居然，竟然”。常用的形容词是：odd, impossible, surprising, shocking, curious, strange, amazing, incredible等。

• It's strange that she should talk to her parents like that.
  真奇怪，她居然那样对父母说话。

(12) 有些名词也能表示类似意思，如：It's a pity/a shame/no wonder/a miracle that...

• It's a shame that he should do such a thing.
  他居然做出这样一件事，真是可耻。
• It is a pity that he should be so conceited.
  他居然这么自负，真是遗憾。

(13) 在现代英语中，有时不用虚拟语气，而用直陈语气。

• It seems incredible that he has made such great progress within so short a period of time.
  在如此短的时间内他竟然取得这样大的进步，这似乎令人难以置信。
• It's strange that he has failed the exam.
  令人奇怪的是，他竟然没通过考试。

(14) 除that从句外，其他从句(主要是名词关系从句)也可以用it作形式主语。

• It doesn't interest me whether you go or not.
  你去与不去我不太感兴趣。
• It doesn't matter much when you arrive — just come when you can.
  你何时到关系不大，你能来时就来吧。
• It's surprising how they are treated there.
  他们在那里受到的待遇真令人吃惊。
• It's a mystery what he was doing while in France.
  他在法国期间做了些什么事仍是一个谜。

(15) it还可以引出带if, as if, as though的句子。在用as if, as though的句子中，主句中谓语动词常是be, look, sound, appear, seem等。

• It looks as if/as though we're going to have trouble with Mrs. Jones again.
  看起来我们又要与琼斯太太产生麻烦了。
• It's not as if this was the first time she'd been difficult.
  这似乎并非是她第一次闹别扭。
• It sounds as if the government doesn't know what to do.
  听上去似乎政府无计可施。
• Very soon it began to appear as if the studies were not progressing smoothly.
  很快研究就好像开始进展得不太顺利了。
• It will be a pity if we have to ask her to leave, but it looks as though we may have to.
  如果我们不得不叫她离开的话，这将会是一件非常遗憾的事情，但现在看来，我们可能不得不这样做。