2.1 Subject

In English grammar, a subject is one of the two main parts of a sentence (the other main part being the predicate). It is the part of the sentence or clause that tells what it is about, or who/what is performing the action (this is called the agent). A subject is almost always a noun, a noun phrase, or a pronoun.

• In a declarative sentence (see post 1.2.1), the subject comes before the verb:

Lee Garner Jr. (proper noun subject) made me (verb) hold his balls (eww!).

• In an intrrogative sentence, the subject comes after the verb:

Is (v) she (pronoun subject) your girlfriend?

• There are exceptions to these common subjects:

Compound subjects are made up of two or more simple subjects joined by a coordinating conjunction (see post 1.2 sentence structure) and share the same predicate.

Miss Calvet and I (proper noun + cc + pronoun = compound subject) are (v) getting married.

An agent is the subject of a sentence formed in the active voice: 

Here to tell that story is (v) Peggy Olson (agent, the one that performs the action).

In a sentence in the passive voice, the agent is the object of a preposition, and the direct object becomes a patient subject:

What you call love (patient subject) was invented (v) by (preposition) guys like me (agent object).
If we flipped this sentence into the active voice, it would read something like this:

Guys like me (agent subject) invented (v) what you call love (patient object).

• If we have no other subject, we use "there" or "it" as a subject, this is called a dummy subject:

Source: http://grammar.about.com/od/rs/g/subject.htm