高中英语句子开头为什么常有It is

高中阶段的语法项目中有主语从句和强调结构用到it is开始的句型.
236 it is／was+形容词+ that… should结构
A that…should结构可以用于 it is／was advisable,better,de－ sirable,essential,imperative,important,natural,necessary之后；也可用于fair（=just）,just,right 之后（这几个词前面常
加 only）以及 reasonable之后,以代替 for+不定式结构：
It is advisable that everyone should have a map．
最好每一个人都有一张地图.
It is better for him to hear it from you．
让他从你那儿得知消息（对他）更好些.
It is better that he should hear it from you．
最好让他从你那儿得知这个消息.
It is essential for him to be prepared for this．
有必要让他对此有所准备.
It is essential that he should be prepared for this．
（译文同上.）
It is only right that she should have a share．
应该有她一份才对.
有时should在be之前可省略：
It is essential that he be prepared．
B that…should 结构可以用在 it is／was absurd,amazing,an－ noying,ludicrous,odd,ridiculous,strange,surprising之类的形容词后面,以代替that+现在时态／过去时态结构：
It is ridiculous that we should be（=that we are） short of water in a country where it is always raining,
在经常下雨的国家里我们竟然会缺水,这简直是荒谬.
说过去的事情时有时用不定式的完成式：
It is amazing that she should have said（＝that she said） nothing about the murder．
令人惊奇的是她对这桩凶杀案竟然不置一词.