A comma is required to separate independent clauses. Place the comma after the coordinating conjunction used to join the two clauses. The seven coordinating conjunctions are: but, so, yet, and, for, or, nor. For example: Annie was running late for work, but she still needed to wash the dirty dishes in the sink.
Use a comma after an introductory word, clause or phrase. Introductory words include however, therefore, well and yes. For example: Therefore, she has to attend summer school in order to graduate on time. Some examples of words that commonly begin an introductory clause are: although, if, because, since, when, after. For instance: Because he got a flat tire, Joe missed his flight. Additionally, put a comma after an introductory phrase. A comma is often necessary after prepositional phrases that contain more than four words, absolute phrases, participial phrases, infinitive phrases and nonessential appositive phrases. Two examples: Having gotten into trouble for talking in class, Zach was forced to miss recess. Lost in a daydream and not paying attention, I tripped on the step.
Use commas to properly frame a quotation. Place a comma before the first quotation mark and before the closing quotation mark. If the quotation ends the sentence, then just use a comma before the first quotation mark. For example: Hearing Robert Frost's quote, "the only way round is through," reminded him to push through the hard times in life. Or: In the words of Robert Louis Stevenson, "marriage is one long conversation checkered by disputes."
Commas are used to separate phrases at the end of a sentence that refer to phrases in the beginning or middle of the sentence. Phrases that are free modifiers can easily be placed in the sentence without causing confusion to the reader. For instance: Crying softly, Sandy watched the moving van drive away. Sandy watched the moving van drive away, crying softly.
Separate three or more words in a series with commas. For example: I bought a fishing pole, tackle box, and lures for the weekend fishing trip. Depending on the style guidelines you are writing in, it may or may not be necessary to use a comma before the coordinating conjunction that ends the series, such as "and."
Use commas to set off geographical locations, like city and state. Example: We went to Orlando, Florida, for vacation last summer. Place a comma before and after a person's name and specific title. Example: Jeff Smith, CEO, will be delivering a speech at the graduation commencement ceremony this year. Put a comma between the day and year in a date, and use a comma to set off an address in a sentence. Just don't put a comma between a street number and name in the address. For example: On August 14, 2009, our address changed to 2145 Main Street, Bentonville, Arkansas.