There are four kinds of sentences...

1. Declarative: makes a statement or tells something; ends with a period.
EXAMPLE:  The sky is blue.

2. Interrogative: asks a question; ends with a question mark.
EXAMPLE:  What is your favorite number?

3. Exclamatory: shows strong feeling; ends with an exclamation point.
EXAMPLE:  I am a big Derek Jeter fan!

Subject: The part of a sentence that names who or what the sentence is about.
EXAMPLE: A tree grows in Brooklyn. 

Predicate: The part of a sentence that tells what the subject does, has, or is like.  The predicate ALWAYS includes a verb.
EXAMPLE: The teased dog growled.

A complete sentence MUST have a subject and a predicate.  It must also express a complete thought.

Complete subject: Includes all of the words in the subject of a sentence.
EXAMPLE: Richie T. Hamster was a good pet.

Complete predicate: Includes all of the words in the predicate of a sentence.
EXAMPLE: Miss Mary Mack was all dressed in black.

Not all of the words in the complete subject and predicate are of equal importance.

Simple subject: the main word or group of words in the complete subject--usually a noun or pronoun.
-Noun: a word that represents a person, place, thing, or idea.  
-Pronoun: a word that takes the place of one or more nouns (such as I, you, he, she, it, we, they).

Simple predicate: the main word or group of words in the complete predicate--always a verb.
-Verb: a word that expresses an action or a state of being.

Main Clause: a group of words containing a subject and a predicate that COULD STAND ALONE as its own sentence
EXAMPLE: Kate loves to dance.
EXAMPLE: She takes hula lessons.

Compound Sentence: a sentence that contains two or more main clauses
EXAMPLE: Kate loves to dance, and she takes hula lessons.

There are rules for the way we join main clauses to form compound sentences.  When these rules are broken, we call this a run-on sentence.

Run-on Sentence: two or more sentences incorrectly written as one
EXAMPLE: Kate loves to dance she takes hula lessons.

To spot a run-on sentence, first determine whether the sentence has two or main clauses.  If there is only one main clause, this is NOT a run-on sentence.  If there ARE two or more main clauses, then look at the way they are joined.  If they are joined with either a comma and a conjunction (and, but, or) OR a semicolon without a conjunction, then this is NOT a run-on.  However, if the main clauses are not joined in one of these ways, then THIS IS A RUN-ON SENTENCE!