When the pronoun it is used as the subject of a sentence it may represent a living being or a thing: then it is a notional subject. Sometimes, however, it does not represent any living being or thing and performs a purely grammatical function: then it is a formal subject.

#1 . When it is a notional subject the pronoun it has the following meanings:

#1.1. It stands for a definite thing or some abstract idea — the personal

The door opened. It was opened by a young girl of thirteen or fourteen. (Dickens)

If this is a liberty, it isn’t going to mean a thing. (Heym)

 #1.2. It points out some person or thing expressed by a predicative noun, or it refers to the thought contained in a preceding statement, thus having a demonstrative meaning — the demonstrative

It is John.

It was a large room with a great window. (Dickens)

Dick came home late, it provoked his father. (Lindsay)

#2. Sometimes the pronoun it is a formal subject, i, it does not represent any person or thing. Here we must distinguish:

(1) the impersonal it; (2) the introductory or anticipatory it; (3) the emphatic it.

a. The impersonal it is used:

to denote natural phenomena (such as the state of the weather, ) or that which characterizes the environment. In such sentences the predicate is either a simple one, expressed by a verb denoting the state, of the weather, or a compound nominal one, with an adjective as predicative.

It often rains in autumn.

It is cold in winter.

It is stuffy in here.

It is delightfully quiet in the night.

NOTE:  The state of the weather can also be expressed by sentences in which the subject denoting the state of things is introduced by the construction there is. In such sentences the noun introduced by the construction there is is the subject.

There was a heavy frost last night.

There was a fine rain falling over the trees, the flowers, and the people sitting on the benches in the garden.

b. to denote time and

It is five minutes past six.

It is morning already.

How far is it from your office to the bank? (Galsworthy)

It is a long way to the station.

NOTE:  Sentences with the impersonal it as subject very often correspond to Russian impersonal one-member sentences.

It is late.

It is freezing.

It is cold.

It is hot.

The following sentences, however, correspond to Russian two-member personal sentences:

It is raining.

It is snowing.

It is hailing.

#2. The introductory or anticipatory it introduces the real

It’s no use disguising facts.

It was curious to observe that child.

NOTE:  Some grammarians treat this it as the real subject and the rest of the sentence as the predicate.

#3. The emphatic it is used for

It was Winifred who went up to him. (Galsworthy)

It was he who had brought back George to Amelia. (Thackeray)