Answer:

Commas may be used to separate the items in a sequence (three or more things). For example, "She went to the store, dropped off the clothes at the dry cleaners, and stopped by the post office."
Use a comma along with a conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses. For example, "She wanted to go to the store, but she did not have a car."
Use a comma to set off introductory elements. For example, "Driving to the grocery store, she suddenly realized that she forgot her purse."
Use a comma to set off parenthetical elements. For example, "The Statue of Liberty, which stands in New York Harbor, was a gift to the United States."