Anticipatory it

it用来充当形式主语或形式宾语，后面的真正主语或真正宾语通常是不定式结构、动名词或名词性分句。It的这种用法叫作先行it。

eg.
It is quite important for us to form a good studying habit.
I think it important to finish the task in time.

it用来充当形式主语，后置的真正主语往往可以取代先行it的位置，出现在句首。