Use a comma to separate independent clauses joined by the coordinating conjunctions "and," "but," "or," "nor," "for," "so" or "yet". A sentence contains more than one independent clause when there are two separate subject and verb agreements, as in the following example: I washed the dishes, and he took out the trash. Never place a comma before a conjunction when a sentence contains one subject and two verbs or verb phrases, as in the following example: I washed the dishes and took out the trash. Similarly, do not place a comma before a conjunction when a sentence contains two nouns, noun phrases or noun clauses acting as the subject or object, as in the following example: The computer at station one and the printer at station three are out of order.
Place a comma after introductory dependent clauses, which usually start with "after," "although," "as," "because," "if," "since," "when" or "while". However, do not use a comma when placing a dependent clause after the main clause. For example, "Because I missed the bus, I was late for class" requires a comma while "I was late for class because I missed the bus" does not. Place a comma after introductory phrases, such as a prepositional phrase longer than four words, as in the following example: To get there on time, we have to leave at 5:00 p.m. . Place a comma after the introductory words "yes," "no," "however" and "well."
Place a comma before and after a nonessential clause, phrase or word. This includes the words "therefore" and "however." A clause, phrase or word is nonessential if it interrupts the flow of a sentence and does not alter the meaning of the sentence if taken out. However, do not use commas to set off essential clauses, phrases or words, particularly clauses beginning with "that," which are always essential. For example, "Ms. Stone, our writing professor, published her first novel last year" requires a comma while "Ms. Stone is the writing professor who published her first novel last year" does not.
Use a comma to separate contrasting elements of a sentence, as in the following example: The necklace belongs to me, not her. Also use a comma to indicate a distinct pause or shift, such as for emphasis or when a sentence contains both a statement and a question. For example, "The speaker seemed timid, even terrified" requires a comma to indicate a distinct pause for emphasis while "You're going to prom, aren't you?" requires a comma to indicate the shift from a statement to a question.
Separate three or more words or phrases in a series with commas, as in the following examples: "I left my purse, keys, and shoes in the car" and "The doctor told me to increase my cardio, lift weights, and eat more fruit and vegetables." Unless you are producing journalistic writing, always use a comma before the final "and."
Use a comma when introducing or interrupting quotations. The following is an example of introducing a quotation: Katherine said, "I have been here before." The following is an example of interrupting a quotation: "Yes," Katherine said, "I have been here before." Also use a comma to separate a quotation from the main discourse when beginning a sentence with a quotation, as in the following example: "I have been here before," Katherine said.
Use a comma to separate two or more coordinate adjectives used to describe the same noun. If you can insert the word "and" between the adjectives, they are coordinate. For example, "I just had the most intense, tiring workout" requires a comma while "I lost my purple cashmere sweater" does not, as the adjectives in the latter example are non-coordinate adjectives.
Place a comma between a city's name and its state and place a second comma after the state name, unless you use the state's two-letter abbreviation. For example, "She lived in Austin, Texas, for five years" requires two commas while "She lived in Austin, TX for five years" requires only one comma. When using an address in a sentence, place a comma between the street name and the city. If you are only using the street number and name, you do not need a comma.
When using a date in a sentence, place a comma between the day of the month and the year and place another comma after the year, as in the following example: She met him on September 1, 2006, in Boston. If you only use the month and year, you do not need to use any commas, as in the following example: She met him in September 2006 in Boston.
Place a comma before and after the name or title of a person being addressed, as in the following example: Thank you, Lauren/Professor, for all your hard work. Place a comma before and after titles or degrees used after a person's name, as in the following example: Jane Doe, Ph.D., is a new professor in the English Literature department. Using a comma before and after "Jr." and "Sr." is optional, but never use commas to set off roman numerals used after names.
Avoid comma splices and run-on sentences. A comma splice occurs when you separate two independent clause with only a comma, as in the following example: My head hurts, I think I have a fever. To correct this sentence, place the correct conjunction after the comma, replace the comma with a semicolon or replace the comma with a period. A run-on sentence occurs when you do not even use a comma to separate two independent clauses, as in the following example: My head hurts I think I have a fever. To correct this sentence, add a comma and the correct conjunction, a semicolon or a period between "hurts" and "I."