当句子的主语是不定式、动名词或主语从句时，常常可以用it作形式主语，而把真正的主语移到句子后部。用it作形式主语的好处是避免句子由于主语太长而显得头重脚轻。it作形式主语在各相关章节都有述及，这里则是做一个归纳，列出it作形式主语的各种英语句型。

1) it作不定式的形式主语a) it + be + 形容词 + 不定式It is difficult to get there before dark. 天黑以前赶到那里很困难。It is better to get up early. 早起会更好。

b) it + be + 名词 + 不定式It was his duty to attend to this matter. 处理这个问题是他的责任。It is the file system's job to manage writing data between the disks and memory. 在内存和磁盘上管理数据是文件系统要处理的东西。

c) it + 动词 + 宾语 + 不定式It takes a lot of effort to recover the system. 恢复系统不是一件容易的事情。It makes me sad to hear that you have to go away. 听说你一定要走，我很难过。

d) it + be + 介词短语 + 不定式It is against the law to steal. 偷窃是犯法的。

2) it作动名词的形式主语a) it + be + 名词 + 动名词 (常用的名词有no good, no use, nuisance, a waste, a wonder 等)It’s a wonder meeting you here! 在这里遇见你真是个奇迹！It’s no use doing a lot of work without thoughtful planning. 没有考虑周到的计划而做一大堆工作是没用的。

b) it + be + 形容词 + 动名词It was very difficult getting everything ready in time. 要及时做好一切准备很难。Is it worthwhile hating each other over such a trifle? 为了这么点事儿，你俩至于结冤吗？

c) 其他情况It doesn’t matter putting these two files together. 把这两份文件放在一起没关系。It doesn’t make any difference my being here. 我是否在这儿并不能改变什么。（注:这里my为动名词的逻辑主语）

3) it作主语从句的形式主语a) it + be + 形容词 + 从句It is clear that the chosen pattern is not the correct one. 显然，所选择的图式是错误的。It is doubtful whether this method is more scientific. 这个方法是否更科学值得怀疑。

b) it + be + 名词 + 从句It is a pity that we shall have to leave tomorrow. 我们明天必须离开，真是遗憾。It is a complete mystery what caused the accident. 是什么引起这场事故完全是个谜。

c) it + 动词 (+宾语或状语) + 从句It matters little who does it so long as it is done. 只要事情做了，谁做都行。It happens that his sister is a closed friend of mine. 碰巧她姐姐是我的一个好朋友。

d) it + 动词的被动语态 + 从句It is said that the rhythm of tides is affected by the moon. 据说潮汐的涨落受月亮的影响。It is estimated that the output will increase by 20% this year. 今年预计产量将提高20%。

e) it + 介词 + 从句It is out of question that China's economy will keep growing for 40 years. 中国经济增长40年应该不成问题。

注: it还可以用于强调句型，详见强调句型一章。

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