An English Handbook for Writers, Students, and Teachers
When two complete sentences are joined by a coordinating conjunction (and, but, nor, for, yet, or, so) put a comma before the conjunction (glue word). Example: I love Taylor Swift’s videos, and I think her music is awesome too.
Use commas for introductory prepositional phrases and similar clauses. When a preposition or conjunction such as in, on, since, at, once, after, although, if, when, while, or until, begins a sentence, put a comma at the end of the introductory phrase. Example: If you get a new iPod, you’ll have to register it. But how do you find the right spot for the comma? Just ask “If what?” The answer, “If you get a new iPod,” is where the phrase ends, and that’s also where the comma goes.
Separate items in a series with commas. Example: Please buy milk, cheese, and eggs at the store.
Set off all interruptions to the flow of the sentence, even names of people and titles of things, with commas. Example: To be a veterinarian, if that’s what you really want to be, you must go to college for seven years. It is possible to read this sentence without its interruption, and it still makes sense. Like this: To be a veterinarian, you must go to college for seven years. This feature tells us this is an appositive, because we have paused for the interruption. Often appositives can be set apart by using parentheses as well.
Phrases that begin with who, whom, whose, which, and that, are surrounded by commas when used in a restrictive way. Example: Miss Thornbuckle, who lives next door, is featured on YouTube this week. (If it’s extra information, use commas. If not, don’t.)
Put a comma after the greeting and closing in a friendly letter. The greeting is usually, Dear Selena, while the closing is generally stated as Sincerely yours, Justin (using the letter writer’s name in place of Justin).
Separate parts of dates and addresses with commas. Example: Monday, May 21, 2012. Another example: I live at 7319 W. State St., in Omaha, Nebraska. However, if you’re addressing a letter or envelope, there’s no comma after St. Instead, drop down to the next line for Omaha, NE 68104 (and don’t write out Nebraska).
Always put a comma between a city and state or a city and country, i.e. Las Vegas, Nevada or London, England.
If you join two sentences with a comma but there’s no conjunction, that’s called a comma splice. Avoid doing this. You must have the conjunction when joining two complete sentences; a comma doesn’t work by itself in this case (but a semicolon might, as you can see).
When a comma follows a title, you’ve got to think about your audience. If you’re publishing online, you’ll probably want to put the comma outside the quotation marks, as this is the British method; however, if you’re publishing for a strictly American (US) audience, you’re supposed to put the comma inside the quotes. Here are examples of each:
(American)My mother asked me if I’d ever heard King Julien sing “I Like to Move It,” and I told her I had.
(British/UK)My mum asked me if I’d ever heard King Julien sing “I Like to Move It”, and I told her I had.