Type “comma rules” into any search engine, and you’ll turn up a multitude of sites. Here are a couple I’ve found that are helpful without going overboard:
The Purdue University OWL (Online Writing Lab)
JPROF Journalism Site
For a quick guide, though, here are a few tips I've found most useful. Commas can be confusing, but if you print this out, tape it somewhere near your computer, and refer to it when you’re in a jam, it should help in many cases.
1. Use commas to set off introductory phrases. Many times, the placement of this comma will occur where you would draw a natural breath if reading the sentence aloud.
As soon as Jenny woke up that morning, she had a sense of foreboding.
After the horse dragged Cowboy Bob five hundred feet, the animal finally slowed to a stop.
Note: Sometimes the intro phrases are short. Use your discretion in those cases. Read the sentence aloud and see if a comma would clarify the sentence’s meaning.
Last night Jimmy snuck into my bedroom after my parents went to bed. (Comma would theoretically go after “night,” but is it needed? Probably not.)
After all wouldn’t everyone rather win a million dollars than drag themselves to a job they hated? (Comma would, and probably should, go after “all” to clarify a natural pause of emphasis in the sentence.)
2. Use commas to separate items in a series. A series equals three or more items, by the way.
Madeline packed a single change of clothes, a few toiletries, food for her pet rat, and enough money to get her across the border.
Note: Placement of the comma before the “and” of the final item has been debated. Some grammar guides will tell you it’s appropriate. Others will tell you it isn’t. My editor recently took out all commas before the “and” in sentences like this. My gut feeling is that it’s a personal preference that won’t make or break your manuscript either way.
3. Use commas to set off interrupting phrases (also called appositives or non-essential/non-restrictive clauses, if you want to get fancy). The test is to see whether the interrupting information is essential to understanding the sentence. Can you drop it out and retain the central thought of the sentence? If the answer is yes, put a comma before and after the interrupting phrase. If the answer is no--if taking out the interrupting phrase will change the sentence significantly or render the sentence grammatically unsound--then you cannot use commas around it.
Sarah Smith, my best friend since first grade, turned out to be the biggest liar I ever met. (The fact that Sarah is the speaker’s best friend might be interesting info, but dropping out the phrase between the commas does not significantly change the fact that she turned out to be a liar.)
The girl who had been my best friend since first grade turned out to be a liar. (Aha! Here, the phrase “who had been my best friend since first grade” is essential to the sentence, because if we take it out, the sentence changes completely, to “The girl turned out to be a liar.” Which girl?? Here, commas may not be used, because every single word is essential to the meaning of the sentence).
4. Use commas before a coordinating conjunction (and, but, or, for, nor, so, yet) when joining two complete thoughts in one sentence:
I wanted to see an actual cowboy who lived in the west, yet I feared flying more than anything else in the world. (The groups of words on both sides of the comma could also function as stand-alone sentences. Therefore, put a comma before the conjunction).
Bryan checked his appearance in the mirror one last time, but he was certain that no one would recognize him in the beard and fake glasses. (Same case as above).
5. Use commas to set off place-names when talking about cities/states/countries. Note that you must place a comma after both the city AND the state/country in the sentence.
Johnny grew up in Dublin, Ireland, but he moved to the United States and settled in Chicago, Illinois, after his mother passed away.
Common Misuses of the Comma - Be Careful!
1. DO NOT use a comma before a coordinating conjunction if it does not separate two complete thoughts.
Dr. Johnson tightened his collar against the wind, and was convinced that the cold would kill him this time. (The second half of this sentence cannot stand by itself, so you cannot put a comma before the coordinating conjunction “and.”)
2. DO NOT use a comma to separate a subject from its verb.
The baseball player, hit a home run in the bottom of the ninth. (Can’t do it! No reason for it! Don’t even try!!)
3. DO NOT use a comma to join together two complete thoughts in a sentence without the proper conjunction between them.
The country road wound along the edge of the woods, no one dared to walk it alone after dark.
(The groups of words before and after the comma could each stand alone and make sense. Therefore, you CANNOT join them with only a comma. You must add a conjunction, or change the comma to a semi-colon--more on this in another WW blog--or simply place a period and make them two sentences.)
The country road wound along the edge of the woods, BUT no one dared to walk it alone after dark.
The country road wound along the edge of the woods. No one dared to walk it alone after dark.
Note: If you are author Joyce Carol Oates, consider all of the above null and void, and use the comma with abandon. If you are not JCO, your agent/editor/publisher will be much happier if you submit a manuscript that does its best to adhere to the above guidelines.
Hope this was helpful to you. Good luck!