Uses of it
It is an impersonal pronoun. It has several uses.
1. To refer to lifeless things and animals:
I have bought a chair. It is made of teak wood.
The dog is barking. It must be hungry.
I bought a new mobile phone last week. It isn’t working.
2. To refer to a young child, except when the sex has to be made explicit:
The baby cried because it was hungry.
3. As a provisional subject at the beginning of a sentence. In this case, it represents the real subject which follows later.
It is useless to wait any more. (=To wait anymore is useless.)
It is true that I wanted to marry her. (=That I wanted to marry her is true.)
4. To refer to some statement in the preceding clause:
He is really responsible for this problem; and he knows it.
He was committing a mistake, and he didn’t know it.
5. As an impersonal pronoun:
It is raining.
It is getting dark.
6. To refer to a collective noun when it is viewed as a single unit:
The committee decided to hold its next meeting on the 25th.
The jury gave its unanimous verdict against the prisoner.
7. It can also be used as a preparatory subject when the real subject is a to-infinitive.
It is easy to find fault with others. (= To find fault with others is easy.)