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Commas 
A comma is like a flashing yellow light or a yield sign—it signals a pause. Just like these traffic signals, the comma should be used in specific situations.
You should use commas in five basic situations in a sentence:
1. Use a comma after an introductory phrase of more than three or four words in a sentence. The comma signals the end of the introductory phrases and the start of the main part or idea of the sentence.
After a prenuptial ceremony in a sweat lodge, the couple gathered with their friends on a mountain top for the wedding.
Tip: The main part of the sentence should be able to stand alone as a complete thought.
2. Use commas at the beginning and end of a phrase that interrupts the main thought. These commas show that the information between them is extra information in the sentence.
The graduate student's paper, in my opinion, was brilliantly written and should be submitted to a journal.
Tip: Put commas around a phrase if you can lift the phrase out of the sentence without changing its meaning.
3. Use commas to separate items in a series.
Jane walked into her teenager's bedroom and found a half-eaten tuna sandwich, a collection of moldy socks, an empty cigarette carton, and a pile of unopened textbooks.
Sean found a half cup of stone-cold coffee, an empty yogurt carton, an open bottle of aspirin, and fourteen stacks of unread reports when he walked into his mother's office.
Tip: In journalism the comma is often omitted after the next-to-last item in a series. In academic writing you usually include the comma.
4. Use a comma before a connecting word to link two complete sentences together.
I could call my boss and grovel for the day off, or I could assert my right to take a mental health day.
Tip: A comma is not strong enough by itself to link two complete sentences together. It must have a connecting word.
5. Use a comma to introduce a quotation in a sentence.
Joanna repeated the medium's message, "You either must make peace with your brother, or you'll be doomed to repeat the same relationship in the next life."
"And I tell you, my fellow Americans," the politician exhorted, "the middle class will not bear the brunt of new taxes."
Commas do have other purposes, and there are more complex reasons for their use in a sentence than what is described above. To understand more about comma use, consult a handbook such as A Writer's Reference (2003, Fifth Edition) by Diana Hacker.
Here are other common other uses of commas:
Use a comma to separate the day from the year in a date.
Tip: When the date is in a sentence and more of the sentence follows, you also need a comma after the year.
I was born on February 18, 1954, in Olean, New York.
Separate a city from a state with a comma.
Tip: When the city and state are in a sentence and more of the sentence follows, you also need a comma after the state.
Empire State College's administrative offices are located in Saratoga Springs, New York, on the same street as the famous race track.
Separate a person's name and title with a comma.
Madeline Albright, Secretary of State
British Prime Minister, Tony Blair
Microsoft CEO, Bill Gates
Tip: When the name and title are in a sentence and more of the sentence follows, you also need a comma after the title.
The agriculture Secretary, Dan Glickman, recalled 25 million pounds of hamburger in August.
The Florida Marlins' World Series triumph was a dream come true for Livan Hernandez, Most Valuable Player.
Use a comma after an introduction to a personal letter.
Dear Susan,
Dear Dr. Belasen,
In long numbers use commas to separate numbers into units of three.
Tip: It's optional to use a comma in a four digit number. According to MLA, use a comma in four digit numbers except in page numbers, line numbers, addresses and years, unless the year has more than four digits, e.g. 10,000 BCE.
Exercise 3: Commas & Semi-Colons
