4 Answers

When a subject is stripped of all the words that describe it, the simple subject is left.
The girls laughed.
The tough girls laughed.
The rowdy, tough girls laughed.
The rowdy, tough girls in the roller derby laughed.
The rowdy, tough girls in the roller derby on TV laughed.
In each of these sentences, the simple subject is girls. A simple subject is only one word.

http://www.ccc.commnet.edu/sensen/part1/three/simple.html

This is your homework.  In this case, "homework is the subject"...

In addition to what Country Bumpkin told you, a subject is generally a noun....a person, place, thing, or idea.  
Girls act very silly around boys they like   (Simple subject: Girls - person/people)
Rhode Island is my favorite vacation place (Simple subject: Rhode Island - place)
Bowling is my hobby!  (Simple subject: Bowling - thing)
Patience is a virtue. (Simple subject:  Patience - idea) 

All.