A sentence is a group of words that expresses a complete thought.
Example: Chrissy won the race. (This is a sentence because it expresses a complete thought. The reader knows who is doing the action and what the action is.)
Non-example: If Chrissy wins the race. (This is not a sentence because it is not a complete thought. The reader knows who is doing the action, but doesn’t know what will happen if the action succeeds. The word “if” causes the sentence to feel incomplete).
A fragment is a group of words that does not express a complete thought. It may or may not have a subject and verb.
Example: Suki is my best friend. (This is a sentence because it expresses a complete thought. The reader knows who it is about and what the author wants the reader to know about Suki.)
Non-example: Suki, who is my best friend. (This is a fragment because it does not express a complete thought. Although the reader knows that Suki is your best friend, he or she is wondering what you want to say about her. The use of the comma and the word “who” tell the reader that there is more information to be read about Suki.)
A run-on sentence occurs when two or more sentences are written as if they are one sentence. Sentences can only be combined by using a connector, such as a conjunction with a comma or a semicolon.
Example: Chrissy won the race, Suki is still my best friend. (This is a run-on sentence because it connects two complete statements with only a comma.)
Non-example: Chrissy won the race, but Suki is still my best friend. (This is a complete sentence because the comma and coordinating conjunction but are used to correctly connect the two ideas.)