Subject

What is Subject?

The subject is the agent of the sentence in the active voice; it is the person or thing that performs or responsible for the action of the sentence, and it normally precedes the verb.
Every sentence in English must have a subject. (In case of commands, the subject [you] is understood).
The subject may be a single noun.
For example,
Coffee is delicious.
Milk contains calcium.
The subject may be a noun phrase.

A noun phrase is group of words ending with a noun. (It can not began with a preposition).
For example,
The book is on the table.
That new red car is John’s.
Examples of subjects:
We girls are not going to the movie.
John likes boats.
Robin, John, Peter, and I went to a restaurant last night.
The weather was very bad yesterday.
The bank closed at two o’clock.

It can act as a pronoun for a noun or can be the subject of an impersonal verb. As the subject of an impersonal verb, the pronoun is not actually used in place of a noun, but is part of an idiomatic expression.
It rains quite often here is the summer.
It is hard to believe that he is dead.

In some sentences, the true subject does not apper in normal subject position. There can act as a pseudo-subject and is treated like a subject when changing word order to a question. However, the true subject appears after the verb, and the number of the true subject controls the verb.
There was (verb- singular) a fire (subject- singular) in that building last month.
Was there (verb- plural) a fire (subject- plural) in that building last month?
Were (verb) there many students (subject) in the room?