it +neccessary\strange结构是什么？

别忘了是有有 be动词 的！
在 “It is +形容词+that……”结构中，使用某些表示愿望、建议、请求、命令等形容词后的主语从句的谓语常用虚拟语气。
其表达形式为it is adj. that ...should ＋动词原形或省略should....
例如：
It's important that we （should可省略, 下同）take good care of the patient.
It’s especially important you (should) be honest.
it is necessary that he (should) be punctual.
你所说的strange也是如此使用
另外，类似的还有：
it is natural that....
it is essential that....
it is urgent that...
it is imperative....
it is (high) time that...
it is a pity/shame that....
it is no wonder that...
后面的从句都要虚拟