You use it to refer to an object, animal, or other thing that has already been mentioned.

You use it to refer to a child or baby whose sex you do not know or whose sex is not relevant to what you are saying.

You use it to refer in a general way to a situation that you have just described.

You use it before certain nouns, adjectives, and verbs to introduce your feelings or point of view about a situation.

You use it in passive clauses which report a situation or event.

You use it with some verbs that need a subject or object, although there is no noun that it refers to.

You use it as the subject of 'be', to say what the time, day, or date is.

You use it as the subject of a link verb to describe the weather, the light, or the temperature.

You use it when you are telling someone who you are, or asking them who they are, especially at the beginning of a phone call. You also use it in statements and questions about the identity of other people.

When you are emphasizing or drawing attention to something, you can put that thing immediately after it and a form of the verb 'be'.

You use it in expressions such as it's not that or it's not simply that when you are giving a reason for something and are suggesting that there are several other reasons.

If you say that someone thinks they're it, you mean that they think they are better or more important than they really are.