I think it important for us to do something 什么情况下is可以省略吗?

I think it important for us to do something
I think it is important for us to do something
都对,前者是it作为形式主语,是一句简单句
后者是 宾语从句,是复合句 I think （that）it is important for us to do something
省略了that