COMMAS
Commas have many uses; some of these include after introductory elements, between items in a series, between independent clauses connected by a coordinating conjunction, and with non-restrictive elements.
After introductory elements:
Use a comma after most introductory elements, such as prepositional phrases, dependent clauses, and some single words.
Dependent Clause: Because of our lead in the market, we may be able to risk introducing a new product.
Prepositional Phrase: Through more careful planning, we may be able to serve more people.
Single Word: Meanwhile, George worked on the written report.
When the introductory prepositional phrase or word is short, you may omit the comma:
Thus we may not need to hire anyone.
Items in a series:
Another use for the comma is between items in a series of three or more. Company style may dictate omitting the final comma, but if you have a choice, use the final comma; doing so can often lessen the chances of misunderstanding.
I brought a pencil, a scantron, and a dictionary to the test.
Between independent clauses separated by a coordinating conjunction (for, and, nor, but, or, yet, so):
She spoke to the sales staff, and he spoke to the production staff.
You may eliminate the comma when the independent clauses are very short.
Non-restrictive elements:
A non-restrictive element, which can be a clause, a phrase, or a single word, contains extra information that is not essential to the sentence; it should be set-off with commas.
Non-restrictive clause: The Time Magazine website, which is produced by Steve Conley, has won several design awards. [the clause is not essential to the meaning of the sentence.]
A restrictive element is one that cannot be omitted without altering the meaning of the main clause; it should not be set off with commas.
Restrictive clause: The website that is produced by Steve Conley has won several design awards. [no commas are used before and after that is produced by Steve Conley because this information is necessary to the meaning of the sentence—it specifies which website.]
Appositives:
Another use for paired commas is to set off appositive words and phrases. An appositive further describes or defines what precedes it. Like non-restrictive clauses, appositives can be dropped without changing or obscuring the meaning of the sentence.
Appositive: Conley, a freelance designer, also produces the websites for several nonprofit corporations.
Between coordinate adjectives:
Commas are used between adjectives modifying the same noun (coordinate adjectives).
She left Monday for a long, difficult recruiting trip.
To test the appropriateness of such a comma, try reversing the order of the adjectives: a difficult, long recruiting trip. If the order cannot be reversed, leave out the comma (a good old friend isn’t the same as an old good friend).