Anticipatory it
it用來充當形式主語或形式賓語,后面的真正主語或真正賓語通常是不定式結構、動名詞或名詞性分句。It的這種用法叫作先行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用來充當形式主語,后置的真正主語往往可以取代先行it的位置,出現在句首。