Question

How can you identify a simple subject from a complete subject?

Answer 1

The simple subject is the single/main word or word group that tells who or what the sentence is about.
Compete subject is more consisting of all the things the subject is about.

EX. Simple subject (capitalized): The RED ANT is on a mission to find food for the colony.
Complete subject (capitalized): The RED ANT is on a mission TO FIND FOOD FOR THE COLONY.

I believe this should help? If not, ask again with more detail.

Answer 2

Try this activity: http://grammar.ccc.commnet.edu/grammar/quizzes/subjector.htm