It's taken me a week to write this article, in part due to formidable college work, in part due to this being the hardest topic I've attempted to cover. By no means do I intend this to be the end all be all of commas, but I hope to give my dear readers a decent sense of how to use them and why to use them.
The modern comma was invented by Aldus Manutius during the renaissance. His purpose was to clarify the sentence, though we have since slightly changed the comma's meaning. When it comes to placing commas nowadays, there are two general schools of thought: One uses them by ear, placing them where natural pauses occur in the sentence. The other uses them according to grammatical guidelines in order to clarify the structure and parts of a sentence. I hold to the grammatical perspective, because it is the only logical way to place commas. Placing commas by ear is subjective and thus prone to inconsistency. Without further ado, let's get to some examples.
When it comes to writing fiction, people think that variation is always better. Everywhere. That means if John said something, Mary replied, he asked, she coughed, he replied again, but she wasn't clear so she inquired (we already used asked), and he reiterated (he already replied), and I interjected to end this. Many writers try to explain too much and avoid using the word said for fear that the reader does not understand. I may not be an expert, but this kind of thinking is wrong. Let's start with an example:
"I love you!" he exclaimed.
"That's good, now help me with the dishes," she replied.
"Don't you love me?" he asked.
"Course I do, now do the dishes," she replied.
"Fine," he sighed.
If there's anything inconsistent in student writing, it's numbers. Do we write 21 or twenty-one, do we say 1234 or one thousand two hundred and thirty four or twelve-hundred thirty-four? What makes this worse is that different sources differ on the rules. For example, in high school, a teacher told me to write out everything under ten, then later I learned to write out anything under one hundred, then later I learned it's a bit more complicated. Allow me to clear things up with a comfortably large set of rules easy enough for any high-schooler to follow.
If a number begins a sentence, write it out. Some style books make an exception for years, and I support this since we're used to seeing years written as numbers.
Incorrect: 12 persons compromise a jury. Nineteen fourteen was a cold and bitter year.
Correct: Twelve persons compromise a jury. 1914 was a cold and bitter year.
As you might guess, asyndeton and polysyndeton are essentially antonyms. Asyndeton is the omission of conjunctions between parts of a sentence, usually closely related, while Polysyndeton is the use of many conjunctions in a sentence. An example of the first: I went to the store, bought some ketchup, drove into a dump truck, landed in the hospital. Notice how there are no conjunctions. This can have the effect of speeding up a passage, forcing the reader to move along without significant pause. The way most people would write this sentence is as follows: I went to the store, bought some ketchup, drove into a dump truck, and landed in the hospital. The final and makes this feel more normal. Now for polysyndeton: I went to the store and bought some ketchup and drove into a dump truck and landed in the hospital. This gives the feel of a run-on sentence, as this and that and those and and and. You get the point. Both of these are poetic effects and should not be overused. Think of them as tools of pacing and melody (read the sentences aloud), not tools to get you an A in English (or Latin).
Let's talk proper quotes from a fiction perspective (I'm sure you can apply this to nonfiction, but that's not my focus).
I've seen too many people write dialogue using horrible inconsistent and, to put it simply, bad quotations. Not that it's hard to look this stuff up, but that's another story. Now let's get started.
When a new character speaks, that character, male or female or nonhuman, deserves a new line.
Incorrect: "How are you?" Mary said. "Stop asking me questions," John said. "You have to get ticked off by everything I say?"
Correct: "How are you?" Mary said.
"Stop asking me questions," John said.
"You have to get ticked off by everything I say?"