INTRODUCTORY IT

A To sing this song is easy.
B It is easy to sing this song.
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 introduce mainly to make the meaning of B easier to understand.
Introductory “it” can fill the position both of the subject and object.

Introductory “it” as a subject:

To watch musical programs is pleasant.
IT IS PLEASANT TO WATCH MUSICAL PROGRAM.

To play football must be fun.
IT IS FUN TO PLAY FOOTBALL.