1)It + be +表语(n. / adj)+ 主语从句
It is still uncertain whether he is coming or not. It is my honor that I have the chance to meet the great writer.
2) It + 不及物动词+ 主语从句
It happens that they were absent. It doesn’t matter whether he will come or not.
3)It + be +done +主语从句
It has been decided that the park will not be open on Mondays.