It is +adj+that的从句
从句用虚拟语气：should + do；其中should 可以省略.
例子：
It is important that you arrive on time.准时到达很重要.
It is necessary that he come next Monday.他有必要下周来.
It is natural that he change his mind.他改变主意很正常.
这种形容词包括 important,necessary,impossible,exxential,imperative,natural,urgent,等,用于表示建议或惊奇等.