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These rules cover the most common comma errors we see in articles submitted to Constant Content.
Use commas after opening clauses in sentences. Unless an opening clause is short and flows seamlessly into the main part of the sentence (“In Vancouver many birds live in trees”), you always need a comma after that opening clause. And if you had used one after that previous example (“In Vancouver, many birds…”), that would have been OK anyway. The lack of a comma after an opening clause really becomes problematic with lengthy opening clauses, such as, “When the birds in Vancouver fly into the trees and perch there for days at a time, people below need to watch out for falling projectiles.” The opening clause is so long in that instance that the reader needs to be alerted that the clause is over and the main part of the sentence is starting. A comma will serve that purpose. (Some writers are tempted to use a semicolon in this situation, but that’s incorrect. We’ll get to that later.) Usually, no comma is necessary when a subordinate clause comes at the end of a sentence: “It rains in Chicago in the summertime” (whereas, if the clause came at the start of the sentence, you’d need the comma: “In Chicago in the summertime, it rains”).
Use commas before verb clauses at the end of sentences: “The man fell on the floor, breaking his glasses.”
Use a comma to separate two independent clauses in a sentence. An independent clause is a part of a sentence that could stand alone if you chose to handle it that way. Example: “The one dog has fleas, and the other dog has mites.” (The same rule would apply in sentences using other linking words, such as “or,” “but,” “while” and “although.” Similarly, the same rule applies with independent clauses and semicolons: “The dog has fleas; the other dog has mites.”)
Do NOT use a comma (or semicolon) when you have an independent clause that’s joined to a subordinate or non-independent clause with an “and” (or other linking word). Example: “The dog with the red tail has fleas and can’t get rid of them.” You don’t use a comma in that situation because the “and can’t get rid of them” part of the sentence can’t stand alone as a sentence. You just don’t need the pause that a comma provides in that sentence, since the clause is so obviously connected to what comes before.
Use a comma to separate items in a series. Example: “The dog has fleas, mites, flies and bees.” In a simple series like that, according to AP Style, do not place a comma before the “and” or “or” unless the items have several words.** If the items in a series have internal punctuation, or a lot of words, then use semicolons to separate the items (and that includes before the “and” or “or” before the final items). Example: “The man owned several dogs, including a beagle with a pronounced limp and pregnant with pups; a German shepherd suffering from hip dysplasia, shingles and gout; and a collie with bad teeth.” It’s OK to just use commas to separate items with several words, if they have no internal commas, but if you have commas within any of the items (as in the German shepherd example above), then you have to use a semicolon to separate each item in the series. If you have several words in each item in a series but aren’t using semicolons to separate them (since they don’t have internal punctuation), then it’s advisable to use a comma before the final item. Example: “The man owned several dogs, including a beagle with a pronounced limp, a German shepherd suffering from hip dysplasia and gout, and a collie with bad teeth.” In that case, you need that last comma, since the word “and” appears in the preceding item, and figuring out when one item ends and another begins is difficult without that last comma. (** In some styles of English usage, including the Chicago Manual of Style, the rule is to always place a comma before the “and” or “or.” If you decide to use this style, make sure you do so consistently.)
Do NOT use a comma when you only have two items in a series. Incorrect: The man ate breakfast at 8 a.m., and lunch at noon.” Correct: “The main ate breakfast at 8 a.m. and lunch at noon.”
Use commas to separate subordinate clauses within a sentence. These are most commonly “which” and “who” clauses: “The boy, who wore a red coat, climbed the tree.” Or: “The red coat, which had five buttons, dropped on the ground.” If you place a comma before a clause such as this, you must place one after it. Don’t use the commas, however, if the clause is essential, meaning the sentence would lose its essential meaning without the clause. For example, in the example above with the boy and the coat, if you were seeking to identify this one boy among many other boys, and intended to use his red coat as the main identifier, then you’d say, “The boy who wore the red coat climbed the tree, while the other boys stayed on the ground.” Or another example with an essential clause: “The box that fell on the ground broke open.” (Usually, essential clauses not involving people begin with “that” and don’t require commas, while non-essential, or subordinate, clauses not involving people begin with “which,” and as a result need commas before and after.)
Use commas to separate appositives within a sentence, but don’t forget to place one both before and after: “Sam Smith, outfielder for the Reds, threw the ball.” Or: “The outfielder with the Reds, Sam Smith, threw the ball.”
Place commas after introductory words in a sentence, such as “Usually,” “Certainly,” “However,” “Therefore,” “Nonetheless” and “Fortunately.”