Many writers just guess when to add a comma, and, as a result, usually use too many. Much of this uncertainty can be changed to confidence when a writer knows a few basic rules.

Use a comma

1. To set off introductory material such as

a) a long descriptive or explanatory phrase
        Example: Depending on the interest rates, we might renew our mortgage for five years.
b) a dependent clause
        Example: If the interest rates are low, we will renew our mortgage for five years.
c) a conjunctive adverb at the beginning of a sentence
        Example: However, we will renew the mortgage for only one year if the interest rate is more than 8 percent.
d) a conjunctive adverb that follows a semicolon in a compound sentence
        Example: The concert organizer has agreed to issue refunds; therefore, you can get your money at the box office tomorrow.

2. To join items such as

a) a horizontal list
        Example: Computers, printers (optional comma here) and fax machines are essential office equipment in the modern office.
b) Two simple sentences with a coordinating conjunction (FANBOYS)
        Example:There won’t be a rematch, so stop pressuring for one.

3. To insert interrupting material such as

a) Explanatory or defining material
        Example: The tallest mountain in the Himalayas, Mount Everest, continues to challenge adventurers from around the world.
b) Dependent clauses
        Example:The Princess of Wales, who had fallen into disfavour with the Royal Family, died in a Paris car crash.
c) Conjunctive adverbs
        Example: The falling stock market, however, rebounded before closing.

4. To separate

a) Consecutive place names (comma before and after names following the first one)
        Example: In Paris, France, an automobile crash took the life of one of the world’s most loving people.
b) A date and a year (comma before and after the year)
        Example: On August 31, 1997, Diana’s harassment by the press came to a violent and fatal conclusion in a tunnel along the Seine River.

5. To quote, put a comma

a) After (or before) the person spoken to
        Example:“Bob, when will you be free?” asked Rita.
b) Between the speaker and the words spoken
        Example:Rita asked, “When will you be free, Bob?”