Punctuation:  Commas

Commas are complicated.  There are lots of different uses for commas--some of them mandatory, some optional.  We'll go through a few of the rules here, but there are many others.  In the interest of time (and reducing confusion), we'll focus on the rules you are most likely to need as you write your fictions. 

Contents of this workshop:
The Four Main Reasons You Should Use a Comma
The Three Main Reasons You Shouldn't Use a Comma

Why You Should:
Reason One:  Always use a comma to separate independent clauses connected with a coordinating conjunction.

Quote :

Hint--an independent clause can stand alone as a sentence.  Coordinating conjunctions are the seven "joining" words.  Some people remember them as FANBOYS:  for, and, nor, but, or,  yet, so.

Example:  Eragon was tired, but he kept fighting. 

We have two independent clauses joined by the conjunction "but."

Reason Two:   Use commas to set off non-essential parts of the sentence.

Quote :

Hint--if you can take the phrase or words out and not change the meaning of the sentence, it's non-essential.
Eragon, a boy, was tired.

It doesn't matter that he's a boy--the message of the sentence is that he's tired.  "A boy" is non-essential.

Reason Three:  Use commas to separate items in a series.

Quote :

Eragon was carrying a knife, a bow, and a sword.
Eragon was carrying a knife, a bow and a sword.

Both of these are actually correct.  I personally prefer the first sentence--there's less chance for confusion.

Reason Four:  Use commas to set off introductory words, phrases, or clauses.[/quote]

INTRODUCTORY WORDS:  These include words like "yes" and "well" and can also include interjections.

Quote :

"Hey, are you going to the store?"
"Yes, I need to buy some food."
"Well, wait a minute and I'll join you."

INTRODUCTORY PHRASES & CLAUSES: 

Under the shade of a dozen apple trees, Eragon felt at peace.  (Introductory phrase)
Because his sword was heavy, Eragon tired quickly.  (Introductory clause)

Note that if the phrase or dependent clause is AFTER the independent clause there is no need for a comma:

Quote :

Eragon felt at peace under the shade of a dozen apple trees.
Eragon tired quickly because his sword was heavy.RIGHT

Reason One:   The Comma Splice

By far the most common error when dealing with commas is the comma splice.  A comma splice occurs when you take two independent clauses (sentences that can stand alone) and "splice" them together with nothing but a comma.

Quote :

Eragon was tired, he kept fighting. 

WRONG
 
Eragon was tired, but he kept fighting. 

RIGHT

 - Here we have joined the clauses with a comma and a conjunction.  You can also join them with a semi-colon (see that workshop for more).

Reason Two:  Separating Subjects From Verbs

Sometimes, people feel the need to insert a comma just before the verb.  This happens most often with state of being verbs like "is."

Quote :

A Rider, is a formidable opponent. 

WRONG

A Rider is a formidable opponent. 

RIGHT

Reason Three:  Separating Verbs

Another frequent problem is the separation of verbs in a compound predicate. 

Quote :

Eragon lunged, and parried with all of his might.  WRONG

Eragon lunged and parried with all of his might.  RIGHT

"Lunged and parried" is a compound predicate.  Don't use the comma.  Here's another example for you:

Quote :

Arya picked, and ate apples.  WRONG

Arya picked and ate apples.  ]RIGHT]

So that's it.  Those are the main reasons that you should and should not use commas.  There are many more rules concerning the use of commas.  It is also important to note that sometimes comma rules may be broken for stylistic reasons--as in the case of introductory prepositional phrases.  However, if you don't know the rule well enough to know if it can be broken--be safe and stick to the basics.