1. Use a comma before any coordinating conjunction (and, but, for, or, nor, so, yet) that links two independent clauses.
Example: "I went running, and I saw a duck."
2. Use a comma after a dependent clause that starts a sentence.
Example: "When I went running, I saw a duck."
3. Use commas to offset appositives from the rest of the sentence.
Appositives act as synonyms for a juxtaposed word or phrase. For example, "While running, I saw a mallard, a kind of duck." "A kind of duck" is the appositive, which gives more information about "a mallard."
If the appositive occurs in the middle of the sentence, both sides of the phrase need a comma. As in, "A mallard, a kind of duck, attacked me."
Don't let the length of an appositive scare you. As long as the phrase somehow gives more information about its predecessor, you usually need a comma.
"A mallard, the kind of duck I saw when I went running, attacked me."
There's one exception to this rule. Don't offset a phrase that gives necessary information to the sentence. Usually, commas surround a non-essential clause or phrase. For example, "The duck that attacked me scared my friend" doesn't require any commas. Even though the phrase "that attacked me" describes "the duck," it provides essential information to the sentence. Otherwise, no one would know why the duck scared your friend. Clauses that begin with "that" are usually essential to the sentence and do not require commas.
4. Use commas to separate items in a series.
Example, "I saw a duck, a magician, and a liquor store when I went running."
5. Use a comma after introductory adverbs.
Example : "Finally, I went running."
Example : "Unsurprisingly, I saw a duck when I went running."
6. Use a comma when the first word of the sentence is "yes" or "no."
Example :"Yes, I saw a duck when I went running."
Example :"No, the duck didn't bite me."
7. Use a comma when directly addressing someone or something in a sentence.
Example : My editor often asks, "Christina, is that article up yet?"
8. Use a comma between two adjectives that modify the same noun.
Example : "I saw the big, mean duck when I went running."
9. Use a comma to offset negation in a sentence.
Example : "I saw a duck, not a baby seal, when I went running."