Many writers--no matter how experienced they may be--struggle to remember when a comma should be used in a sentence. Commas seem random and illogical at first, but, once you know some basic rules, they start to make more sense. Here are the most important things to remember:
1. A comma separating two independent clauses (complete sentences each with their own subject and verb) creates a comma splice (because the comma incorrectly splices together two complete sentences). For example, the following is a run-on sentence: Sara works at the Writing Center, she likes commas. The part of the sentence on each side of the comma is an independent clause and could stand alone as its own sentence.
2. If the same two clauses were merged without a comma, they would create a run-on sentence (Sara works at the Writing Center she likes commas). A run-on sentence is not necessarily long; this label simply means that the sentence runs on past the place where it should be punctuated.
3. So, if a comma creates a comma splice, but removing the comma creates a run-on, how are you supposed to punctuate that sentence? Two independent clauses can be joined by a comma if the comma is paired with a conjunction. The conjunctions that like to hang out with commas are called FANBOYS. This acronym stands for seven conjunctions: for, and, nor, but, or, yet, and so. A conjunction signals a relationship between two clauses and allows them to be linked with a comma. This sentence is correct and not a run-on: Sara works at the Writing Center, and she likes commas. If you don’t want to use comma-FANBOYS, you can use a semicolon or just separate the sentences with a period.
4. If your sentence starts with a dependent clause (contains a subject and verb but expresses an incomplete thought) and is followed by an independent clause, it must separate these clauses with a comma. Here’s an example: After Mark poured his cereal, he ate it. The comma in this type of sentence is like a “to be continued.” It indicates that a complete thought will follow the incomplete thought.
These are some of the trickiest ways in which commas are used, and I hope they make more sense to you now. If you need more advice on how to use commas, check out the resources on the Writing Center’s website.