形式主语it引导的主语从句
有时为避免句子头重脚轻，常用形式主语it代替主语从句作形式主语放于句首，而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下：（1） It + be + 名词 + that从句
It is common knowledge that 是常识
It is a surprise that 令人惊奇的是
It is a fact that 事实是
（2）It + be + 形容词 + that从句
It is necessary that 有必要
It is important that 重要的是
It is obvious that 很明显
（3）It + be + 动词的过去分词 + that从句
It is believed that 人们相信
It is known to all that 众所周知
It has been decided that 已决定
（4） It + 不及物动词 + that 从句
It appears that 似乎
It happens that 碰巧
It occurred to me that 我突然想起
另注意：在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时，谓语动词要用虚拟语气“(should) +do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that
It is suggested (requested, proposed, desired, etc.) that
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