A complete sentence (an independent clause) is not merely a group of words with a capital letter at the beginning and a period or question mark at the end. A complete sentence has a subject (the actor in the sentence) and a predicate (the verb or action of the actor). However, that's not the only thing required to make a sentence complete. It must also have a complete thought--it can stand alone, with no other sentences around it, and make sense.

As your sentences grow more complicated, it gets harder to spot and stay focused on the basic elements of a complete sentence. You may think that since all of your sentences have capital letters and end punctuation, you don't have any problems. That's where run-ons, comma splices, and fragments can pop up. A good basic way to spot many of these errors is to find and circle the subject and predicate in your sentences.