Introductory "It"

Structure: It + verb + subject complement + infinitive phrase (real subject)


A : To get the best score is hard.
B : It is hard to get the best score. 

In this pattern, it has no meaning. It is used only to fill the subject position in the sentence. Thus, it is called introductory “it”. 
A and B mean the same thing, but sentence B is more common and useful than A. A was introduced mainly to make the meaning of B easier to understand. Introductory “it” can fill the position both of the subject and object.

When the subject is a clause :
It does not matter whether he comes or not. (Whether he comes or not does not matter.)

Introductory it as an object :
Do you think it dangerous to drive when the way is slippery?

Introductory it in question
Who was it that broke the window?