use commas to seperate items in a series

ex:Dwayne went to the store and bought milk,eggs,cheese,and vegetables to make an omelette.

use a comma before a coordinating conjunction when it joins independent clauses in a compound sentence.

ex: Neither Helen nor Rachel understood the lesson,but  they both got an A on the test.

Use commas to set off an expression that interrupts a sentence, non essential participap phrases, nonessential appositives, and words of direct adress parenthical clauses

ex: The bunny, hopping across the field, had a flower in its mouth

Use a comma after a certain introductory elements

ex: To keep your bones strong, be sure to get regular excersise and eat foods rich in calcium.

use commas in a certain conventional situations

-seperate items in dates and addresses

-after a salutation of a personal letter and after closing a letter

ex: On March 17, 2011 everyone wore green to school