I believe this is the biggest technical problem people have. Run ons are expected, but having too many commas is just as bad. I've consulted wikipedia for a few definitions and examples. I hope it's pedagogically useful or just brings up errors that you find obvious.

Punctuation gives organization, intonation, and pauses in your writing.

Read your writing, saying out the words in your mind. You know exactly what you meant, unlike every other person that will go through your work. Is there something your mind lingers on interpreting? That's a good hint you need to rewrite the sentence because everyone else will only find it more difficult than you.

Common technical causes include: you've tacked on too many prepositional phrases, appositives, or subordinate clauses (run on). Sentences shouldn't go on for so long that your mind starts to drift.
Sometimes what these phrases, clauses, and pronouns refer to is unclear (misplaced modifiers). These clauses and phrases usually follow words like "that" , "which", and "who."
You've used improper punctuation that throws the reader off. There's a caveat in that punctuation for dialogue differs from narration. The spoken word comes from the top of the head, so it tends to be more abrupt and most of the following won't apply. The rule of thumb is: if it's something that people would say to each other, then it's okay. In fact you shouldn't talk like you write. That's how some dialogue gets notoriously long-winded. If you are trying to write Dumbledore, oratory is a separate art so you are probably messing up if you make him talk like you write just because it's more complicated.

The biggest issue is that there are a lot of commas improperly placed between independent clauses without a conjunction. "I came, I saw, I conquered" is an example of asyndeton, a device which deliberately omits conjunctions. It speeds up the rhythm--I think of it as useful for making a series of closely related clauses sound very affirmative and sure. Asyndeton without parallel structure doesn't work.

What is parallel structure? "Compare the following examples:
Not parallel: Mr. Thomas likes cooking, jogging, and to read.
Parallel: Mr. Thomas likes cooking, jogging, and reading.
In the above example, the first sentence has two gerunds and one infinitive. To make it parallel, the sentence was rewritten with three gerunds instead.
Not parallel: Affy ran across the yard, jumped over the fence, and down the alley he sprinted.
Parallel: Affy ran across the yard, jumped over the fence, and sprinted down the alley."
http://en.wikipedia.org/wiki/Parallelism_(grammar)

Also asyndeton every other sentence is not a device but a mistake called a comma splice.

Simply removing the comma does not correct the error, but results in a run-on sentence. There are several ways to correct this:

WRONG: It was nearly half past five, they couldn't reach town before dark.

Change the comma to another punctuation mark:
It was nearly half past five; they couldn't reach town before dark.
It was nearly half past five – they couldn't reach town before dark.

Write the two clauses as two separate sentences:
It was nearly half past five. They couldn't reach town before dark.

Insert a coordinating conjunction following the comma:
It was nearly half past five, and they couldn't reach town before dark.
It was nearly half past five, so they couldn't reach town before dark.
(Notice these two conjunctions totally give specific meanings to the sentences, so revise carefully and don't throw in a conjunction just to correct the error.)

Make one clause dependent on the other:
As it was nearly half past five, they couldn't reach town before dark.
Because it was nearly half past five, they couldn't reach town before dark.

Use a semicolon or dash and a conjunctive adverb:
It was nearly half past five; therefore, they couldn't reach town before dark.
It was nearly half past five – therefore, they couldn't reach town before dark.
What's an independent clause? You got a few obvious conjunctions: and, nor, but or, yet, and so. If you remove the conjunction and the resulting group of words makes sense as its own sentence, that's an independent clause.

The second biggest issue is there are commas improperly placed in a compound predicate. This is not necessary unless the predicate phrase is lengthy and the sentence meaning is obscured without it or taxing to read. It's as bad as a run on because the stop is too long like a brain hiccup. As shown above, compound predicate without parallel structure also causes a hitch when reading.

WRONG: "He gathered wood from the forest, and stacked it neatly."
RIGHT: "He gathered wood from the forest and stacked it neatly."

"Commas are often used to enclose information which is not essential to the meaning of the sentence. Such phrases are both preceded and followed by a comma, unless that would result in a doubling of punctuation marks, or the parenthetical is at the start or end of the sentence. The following are examples of types of parenthetical phrases:

Introductory phrase: Once upon a time, I didn't know how to use commas.
Address: My father ate the bagel, John.
Interjection: My father ate the bagel, gosh darn it!
Aside: My father, if you don’t mind my telling you this, ate the bagel.
Appositive: My father, a jaded and bitter man, ate the bagel.
Absolute phrase: My father, his eyes flashing with rage, ate the bagel.
Free modifier: My father, chewing with unbridled fury, ate the bagel.
Resumptive modifier: My father ate the bagel, a bagel which no man had yet chewed.
Summative modifier: My father ate the bagel, a feat which no man had attempted."

Notice none of the parts set off with commas are independent clauses. They do not have a subject and predicate. "Flashing with rage" and "chewing with unbridled fury" are actually acting as adjectives. Address and interjection are difficult to mess up, but a few of the rest can be tricky.

http://en.wikipedia.org/wiki/Comma
"In lists," "Between adjectives," "Emphasis and clarity," and "Elision" mention some subtle uses.

You should strive to write a mix of simple, compound, and complex sentences. However considering you want people to read your writing for pleasure, your complex sentences should not grow into monsters. If you're in doubt, split the sentence or ask someone else to help you reorganize the sentence.

 

Why on earth would you would rely on Wikipedia for such a fickle subject of comma use is beyond me. With the amount of crap on that website makes it only good if you didn't know what a comma was in the first place.

I see no benefit of this post anyway - as far as I can see all you've done is copy/paste from Wiki articles.

Besides, most people have a lot more to worry about than comma usage in their stories - and the ones that really need it won't even see this thread.

WBM Grammar nazi?

 

I'm not sure what the impetus for the sudden comma lecture was, but I'll just point out that the information in the article is accurate, wikipedia or no.

 

I agree with Sesc to an extent; I use whatever punctuation sounds right to me. However, that first example really is wrong. You don't separate two sentences with a comma unless you use a conjunction. For example, you would use

This is slightly more grammatically correct, but it still doesn't sound right. The other three choices are still better. The way I see it, a semicolon is a comma for separating sentences instead of parts of a single sentence. They have essentially the same meaning but are used in specific situations.

 

I wrote this because it's the place for it, and these mistakes are some of the most common. I used Wikipedia to help me enumerate functions, not because I don't understand what they are. I've supplemented them with notes, and tried to include the most important info while leaving out the rest. If you take issue, we can discuss it.

Sesc: We are taught grammar rules and then learn to break them. I agree asyndeton is a perfectly valid device. I suggest people use it when their sentence has parallel structure, and I claim your story shouldn't be made up of it. There are many more people that comma splice without really thinking of the feel it lends. Reading a whole paragraph (or even a huge sentence) of it makes you feel like you're running out of breath.

"(Notice these two conjunctions totally give specific meanings to the sentences, so revise carefully and don't throw in a conjunction just to correct the error.)"
This applies to all the methods of punctuation you can revise a comma splice with--maybe I should elaborate. Reading in your head what you write to hear how it flows is the best way without another person.

"because the stop is too long, like a brain hiccup."
"because the stop is too long like a brain hiccup."
There are two different ways to say this. The first sets off "like..." as an aside, while the second says it all in one breath. This is stylistic; I dislike the comma if the phrase is short.

However, I really take issue with:
WRONG: "He gathered wood from the forest, and stacked it neatly."
RIGHT: "He gathered wood from the forest and stacked it neatly."

Again, you might claim it's justifiable, but if so it's less so than your "veni, vidi, vici." Reading through a paragraph of that is annoying because of having to stop so much.

 

As a repeat comma offender, I offer in my defense the following.

I tend to use them for flow control. Forced flow control is likely bad writing style, but it's something (fanfiction) I do for fun and in it's own way, my story being told. So I feel I have leeway.

If I had a solid, consistent beta that had a better grasp of grammar than I, or a professional editor, I'd likely have this impulse purged. My worst sin in this situation is that I know it's wrong, but continue to do it regardless.

 

Irony lulz:

<_<

Someone should take advice from their own post.

 

I usually have a comma before I use "but", but maybe that's because of the way I talk. The way I see it, when you use "and", you're separating two related ideas that follow from each other. "But" separates two ideas that are usually not as closely related because the second idea doesn't directly follow from the first. For example:

Personally, I think it sounds better to pause before I use a "but".

 

As far as I'm aware, putting commas where you would pause in speech is only a rough approximation used to help people figure out where to put them, rather than being a grammatical rule.

 

I never tried to claim it was a grammatical rule. Actually, iirc, the issue of commas before conjunctions is a matter of personal taste.

From this website: