I've been published by many different houses in my time as an author, and one thing that has never been consistent is how you're supposed to use commas. I've had editors remove the ones I've put in, and others have me add a zillion. I try to follow the guidelines I've found and printed, but they don't always work…still they make sense to me and I'm going to share them with you.
1. Use a comma between two clauses that stand alone and are joined by a coordinating conduction (and, but, or, nor, for, yet, or so…)
Example: Jane and Bob traveled for hours on their trip to Yellowstone, and they arrived exhausted.
DON'T USE A COMMA: Jane and Bob traveled for hours on their trip to yellowstone and arrived exhausted.
If you notice…there are not two in depended clauses in the second example.
2. Commas set of introductory statements or words from a stand alone clause:
Example: Running her hand through her unbrushed hair, she imagined how frightful she looked.
Example of word: However, she wore fresh makeup.
3. Commas separate items listed in series (whether words, phrases, or clauses.)
Example: My favorite types of food are Mexican, Italian, and Greek.
2nd Example: I love writing westerns because the men were brave, the women were strong, and the era interests me.
3rd Example: Some people like Opera, some like jazz, but my sister loves Rock and Roll.
4. Surround a "non-essential" phrase. These phrases can be removed and not alter the meaning of the sentence.
Example: San Francisco, which is not far from Sacramento, was once hit by a major earthquake.
Note: If you remove the part about Sacramento, the sentence still makes perfect sense.
5. This one is tough for me to remember. Use commas about nonrestrictive describers; words that don't limit the meaning of the words they describe to a specific thing, place, etc:
Example: People, who have hearty appetites, are not always overweight.
6. The easiest way to to decide on commas:
a. Use them to set off sentence interruptions
b. Use them with names, degrees, titles, addresses, numbers and dates.
c. Use commas to separate adjectives. Example: She slept under a warm, snuggly comforter. Rule of Thumb: If two or more adjectives precede a noun, you can decide by trying to add "and" rather than a comma. If it sounds good, then use the comma. An example of a time when you wouldn't use one: The cold cement floor provided no comfort.
d. Above all, use a comma to clarify a sentence, and I'm going to borrow the sentence verbatim from the site I use for reference:
As he was leaving, Tom said he would never return.
Reason: without the comma, the reader might think "he" was leaving Tom. Of course, then if you use this sentence, you editor would probably argue "passive voice" and want you to change it to read…
As he left, Tom said he would never return. That pesky "was"…you never know when to use it and when not to. *smile*
Example: Sue claims Andrea is an expert gardner. Sue, claims Andrea, is an expert gardner.
Reason: Both sentences are right but mean different things. In the first, Sue claims Andrea is an expert gardner, in the second, Andrew claims Sue is the expert.
I hope you find this as useful as I do, but then it all depends on your editor and the house rules. BTW…I want to credit http://writingcenter.byu.edu/handouts/PunctuationUsage/commas.html which I notice has changed since I printed off my copy, and they've
added more information to confuse you. :)