Use a comma to set off independent clauses (in which each phrase is a complete sentence): She took the test, and she passed. Do not use to set off dependent clauses (in which one or more of the phrases is a fragment): She took the test and passed. Do not use a comma to insert a “breath” or ungrammatical pause in a sentence.
Use a comma for clauses beginning with the word which: He took the course, which met every Tuesday night. Do not use for phrases beginning with the word that: She took the course that fit her schedule. Do not use a comma before “Jr.” or Roman numerals that are part of the name.
In general, do not use the serial comma (the comma following the second-to-last item in a series). When the absence of a serial comma compromises clarity or when the items are complex phrases, you may use the comma, even though it has not been used throughout the remaining text.
Use a comma to separate a series of adjectives equal in rank (meaning you can replace the comma with the word “and” without changing the adjectives’ meanings): She was a helpful, thoughtful student. Do not use a comma when the last adjective before a noun is an integral element of the noun phrase: He was a smart high school student; She wore a robin’s egg blue parka.
Use a comma to introduce a complete-sentence quotation within a paragraph: Rao said, “The Quest for Distinction is important to VCU.” Use a colon to introduce quotations of more than one sentence: Rao said: “The Quest for Distinction is important to VCU. It received approval from the faculty in May 2002.” If attribution follows a quotation, use a comma inside the quotation marks unless the quotation is a question: “This is an important time for VCU,” Rao said. (but, “Can you send me your address?” he asked. ) If a quoted sentence is broken up by an attribution, use a second comma to indicate the interruption and continue the quote in lowercase unless necessary: “During my time at VCU,” Rao said, “the university has worked to maintain a culture of excellence.”
Use a comma when directly addressing a person or people in print: Classmates, please send me news; No, Ann, I did not get your letter. When referring to a family member or friend of another person, do not use commas around that family member or friend’s name unless you know the other person has only one such family member or friend: Bob’s daughter Sarah; Jane’s professor Chris; But: Bob’s eldest daughter, Sara; Jane’s English 101 professor, Chris.
Use comma before and after state abbreviation. Example: Fredericksburg, Va., is a pretty great place. However, not after postal codes in addresses: Richmond, VA 23224
Place a comma after all dates that include the year. Example: According to her June 10, 2017, letter, Amy will be here in August. Commas do not separate seasons and months (without dates) from their years. Example: The report will be released in August 2018.