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.) thatIt is suggested (requested, proposed, desired, etc.) that
