The rules for punctuating in proximity to simple coordinating conjunctions (and, but, and or) are straightforward, but writers can become confused about when and where to put a comma or other punctuation. Sentences that illustrate the basics, and a discussion of each, follow.
When a conjunction links an independent clause (one that could stand on its own as a sentence) with a dependent clause (one that would form an incomplete sentence), omit internal punctuation: “She went to the pool and took a swim,” or “She went to the pool but did not swim.” (“She went to the pool” is an independent clause; “took a swim” and “did not swim” are dependent.)
When a conjunction links two independent clauses, precede it with a comma: “She went to the pool, and then she took a swim,” or “She went to the pool, but she did not swim.” (“Then she took a swim” and “she did not swim” are independent clauses.) The comma may be omitted for a very short compound sentence such as “She swam and then she ate”; longer sentences, such as the examples given earlier in this paragraph, are often written without internal punctuation, but doing so is not recommended, and for consistency, even brief sentences with two independent clauses should include a comma.
A comma should not follow a conjunction unless it is the first of two commas framing a parenthetical phrase, as in “She went to the pool and, soothed by the cool water, took a swim” and “She went to the pool but, chilled by the cold water, did not swim.” (Said another way, don’t precede a verb with a comma unless the comma closes a parenthetical.) These commas bracket “soothed by the cool water” and “chilled by the cold water,” respectively, which, when omitted, leave the sentences “She went to the pool and took a swim” and “She went to the pool but did not swim,” which do not require internal punctuation.
A pair of em dashes or parentheses can take the place of the two commas: When the parenthesis is abrupt or provocative, use em dashes, as in “She went to the pool and—undeterred by the piranhas—took a swim.” When the interruption is subtle or offered as an aside, use parentheses, as in “She went to the pool and (though she felt tired) took a swim.”
Although older literature, especially that written in British English, can be found that employs both a semicolon and a conjunction to provide a stronger contrast between two independent clauses (“She went to the pool; but she did not swim”), this is now considered incorrect, because the semicolon and the conjunction are redundant to each other. If you use a semicolon, the sentence should be rendered as follows: “She went to the pool; she did not swim.” Use of a comma in place of a semicolon in such a sentence is incorrect; this error is called a comma splice.
Sentences in which or connects clauses should follow the same guidelines: “She went to the pool or the beach,” “She went to the pool, or she went to the beach,” “She went to the pool or, depending on the weather, the beach.”