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