subject

The subject of a sentence is the person or thing doing the action or being described. For example (subjects shaded):

·         Lee ate the pie.

(Lee is the subject of the sentence. Lee is the subject of the main verb ate; i.e., Lee is the doing the action.)

·         Lee is putting on weight.

(Lee is the subject of the sentence. Lee is the subject of the main verb is; i.e., Lee is being described.)

The subject of a sentence is one of the basic parts of a sentence. The other basic part is the predicate. The predicate tells us something about the subject (i.e., it tells us what action the subject is performing, or it describes the subject). Every sentence must have a verb, and every verb must have a subject. 

Read more about predicates.

Simple Subject, Complete Subject, and Compound Subject

The subject of a sentence will be a noun or a pronoun (including all the modifiers that go with it). For example:

·         Pierre puts a lot of garlic in his food.

(Pierre is the subject, and puts a lot of garlic in his food is the predicate. This is an example of a simple subject. A simple subject is just one word without any modifiers.)

·         That boy puts a lot of garlic in his food.

(That boy is an example of a complete subject. It is the simple subject (in this case, boy plus all modifiers.) 

·         That new boy from Paris puts a lot of garlic in his food.

(That new boy from Paris is a complete subject. It is the simple subject (boy) plus all modifiers.) 

·         Pierre and Claudette put a lot of garlic in their food.

(Pierre and Claudette is a compound subject. That just means it is made up of more than one element.) 

·         That new boy from Paris and the tall girl with the long hair put a lot of garlic in their food.

(That new boy from Paris and the tall girl with the long hair is a compound subject made up of two complete subjects.)

A complete subject will be a noun phrase or a noun clause.

Subjects in Different Sentence Structures

The typical sentence structures are:

The subject performs an action:

·         My dog bit the postman.

The subject is described:

·         My dog is boisterous.

(When the subject is being described, the verb (in this case, is) will be a linking verb.)

The subject is identified:

·         My dog is the one in the middle.

(When the subject is being identified (which is just another way of being described), the verb will be a linking verb.)

The subject has an action done to it:

·         My dog was taken to the vet.

(When the subject has an action done to it, the sentence is called a passive sentence.)

http://www.grammar-monster.com/glossary/subject.htm