Definition and Function

The comma’s role is to provide clarity
for a reader and to provide a sense of direction for your sentences.
The comma is a slight break or pause, suggesting a transition of some
kind.
Comma usage is in some respects a question of personal writing
style: some writers use commas liberally, while others prefer to use
them sparingly. Most modern North American style guides now recommend
using fewer commas rather than more, so when faced with the option of
using a comma or not, you may find it wise to refrain.

Rules for Comma Usage

Use a Comma to:

Explanation

Example

Use a comma to separate the elements in a series

You may have learned that the comma before the "and" is unnecessary,
which is fine if you’re in control of things. However, there are
situations in which, if you don’t use this comma (especially when the
list is complex or lengthy), these last two items in the list will try
to glom together (like macaroni and cheese). Using a comma between all
the items in a series, including the last two, avoids this problem.

"He hit the ball, dropped the bat, and ran to first base."

Use a comma + a little conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses,

Contending that the coordinating conjunction is adequate separation,
some writers will leave out the comma in a sentence with short,
balanced independent clauses If there is ever any doubt, however, use
the comma, as it is always correct in this situation.

"He hit the ball well, but he ran toward third base."

Use a comma to set off introductory elements

It is permissible to omit the comma after a brief introductory element
if the omission does not result in confusion or hesitancy in reading.
If there is ever any doubt, use the comma, as it is always correct.

Running toward third base, he suddenly realized how stupid he looked."

Use a comma to set off parenthetical elements

A "parenthetical element," means a part of a sentence which can be
removed without changing the essential meaning of that sentence. The
parenthetical element is sometimes called "added information." This is
the most difficult rule in punctuation because it is sometimes unclear
what is "added" or "parenthetical" and what is essential to the meaning
of a sentence.

"The Founders Bridge, which spans the Connecticut River, is falling down."

Use a comma to separate coordinate adjectives

If you can put an and or a but between the adjectives, a comma will
probably belong there. For instance, you could say, "He is a tall and
distinguished fellow" or "I live in a very old and run-down house." So
you would write, "He is a tall, distinguished man" and "I live in a
very old, run-down house." But you would probably not say, "She is a
little and old lady," or "I live in a little and purple house," so
commas would not appear between little and old or between little and
purple.

That tall, distinguished, good looking fellow."

Use a comma to set off quoted elements

Because we don’t use quoted material all the time, even when writing,
this is probably the most difficult rule to remember in comma usage. It
is a good idea to find a page from an article that uses several
quotations, photocopy that page, and keep it in front of you as a model
when you’re writing. Generally, use a comma to separate quoted material
from the rest of the sentence that explains or introduces the
quotation:

Summing up this argument, Peter
Coveney writes, "The purpose and strength of the romantic image of the
child had been above all to establish a relation between childhood and
adult consciousness."

Use commas to set off phrases that express contrast

Some writers will leave out the comma that sets off a contrasting phrase beginning with but.

Some say the world will end in ice, not fire.

Never use only one comma between a subject and its verb.

Although readers might pause after the word "oneself," there is no reason to put a comma there.

"Believing completely and positively in oneself is essential for success."

Typographical Reasons

Between a city and a state [Hartford, Connecticut], a date and the year
[June 15, 1997], a name and a title when the title comes after the name
[Bob Downey, Professor of English], in long numbers [5,456,783 and
$14,682], etc.

Superfluous Commas

Rules for Not Using Commas	Example
Do not use a comma to separate the subject from its predicate.	WRONG] Registering for our fitness programs before September 15, will save you thirty percent of the membership cost. [RIGHT] Registering for our fitness programs before September 15 will save you thirty percent of the membership cost.
Do not use a comma to separate a verb from its object or its subject complement, or a preposition from its object	WRONG] I hope to mail to you before Christmas, a current snapshot of my dog Benji. She traveled around the world with, a small backpack, a bedroll, a pup tent and a camera. [RIGHT] I hope to mail to you before Christmas a current snapshot of my dog Benji. [RIGHT] She traveled around the world with a small backpack, a bedroll, a pup tent and a camera.
Do not misuse a comma after a coordinating conjunction	[WRONG] Sleet fell heavily on the tin roof but, the family was used to the noise and paid it no attention. [RIGHT] Sleet fell heavily on the tin roof, but the family was used to the noise and paid it no attention.
Do not use commas to set off restrictive elements	WRONG] The fingers, on his left hand, are bigger than those on his right. [RIGHT] The fingers on his left hand are bigger than those on his right.