Comma splices typically result when a writer uses a comma instead of a semicolon to combine two independent clauses in a compound sentence. These sentences contain at least two independent clauses, such as "I enjoy eating pizza" and "pepperoni is my favorite topping." When a writer simply uses a comma to separate these two independent clauses, he is guilty of using a comma splice, as in: "I enjoy eating pizza, pepperoni is my favorite topping." To be correct, this sentence requires a semicolon, as "I enjoy eating pizza; pepperoni is my favorite." Or, the writer can use a coordinate conjunction like "and" or "but" to create the less formal sentence "I enjoy eating pizza, and pepperoni is my favorite topping."
Run-on sentences can either have incorrect punctuation or none at all, resulting in misleading or ambiguous statements, as: "I often go to the store I like to buy my groceries with coupons." It is unclear whether the writer is describing a store he likes, or how he likes to buy his groceries with coupons. Such a run-on sentence may need to be rewritten entirely to clear up the ambiguity, as "I like the store where I often go to buy my groceries with coupons."
Clauses in a sentence can either be dependent or independent. An independent clause can stand on its own as a sentence: "I enjoy the movies." A dependent clause cannot stand on its own as a sentence and is considered a fragment on its own: "When I go to the movies." Clauses beginning with "when" are often followed by a comma, as: "When I go the movies, I always buy popcorn."
You can always separate two independent clauses with a semi-colon. Many writers erroneously use a comma instead, thus creating a comma splice. You could also include a coordinate conjunction after the comma, remembering them by the "fanboys" mnemonic: “for,” “and” “nor,” “but,” “or,” “yet” and “so.” Thus, the comma splice “I like to drive, I spend a lot of money on gas” can be corrected as: “I like to drive, so I spend a lot of money on gas." You can also fix a comma splice by replacing the comma with a semi-colon and using the word “however” followed by a comma. So, “I watch TV at night, my dog hates the noise” would turn into, “I watch TV at night; however, my dog hates the noise."
To lessen the possibility of comma splices or run-on sentences, consider dividing sentences with more than one clause into separate sentences. To decide whether this is a good idea, think about your audience and purpose. If you are writing an instructional piece that gives basic information or directions, using shorter sentences may be more effective. Longer, complicated sentences work well in more formal essays or dissertations.