Appositives are words or short phrases that define another word. If the appositive can be eliminated, it is non-essential and can be set off with commas. "Waldo's wife, Hortense, came from Cordoba." Waldo has only one wife, so her name can be set off with commas.
When the appositive is essential, it is never set off with commas. "Waldo's sister Imogene is taller than he is." Waldo has more than one sister.
A comma separates two independent clauses connected by a conjunction: and, but, so, or, nor, while, yet. Each half of the sentence, or each clause, must have a subject noun or pronoun and a verb and must be able to stand alone as if there were no comma and conjunction. "We went to bed very late, so we slept long past lunchtime."
Short clauses need not be separated by a comma. "We wanted to stay but we had to leave."
If there is no conjunction, don't separate the clauses with a comma. The incorrect joining in "Rosalie came to the party, Claramae stayed home " is a "comma splice." Instead, link them with a semicolon. "Rosalie came to the party; Claramae stayed home."
When two clauses share the same subject, don't separate them with a comma. "I am going to Southgate and plan to see my old home."
When a clause or phrase begins a sentence, a comma should set it off. "Seeing snow piling up, the boss sent us home two hours early." A brief introductory phrase doesn't need a comma. "After lunch I took a nap."
An essential clause or phrase can't be left out of a sentence without altering the sentence's meaning. It is never set apart with commas. "Children who disobey their parents risk being punished." Incorrectly setting off "who disobey their parents" with commas suggests that all children should be punished.
A non-essential clause or phrase can be dropped without changing the meaning or leaving out essential information. Commas can set it off without changing the meaning. "The trees, turning colors, were unusually gorgeous this October."
A comma separates adjectives when they modify the same word. "This is a boring, tiresome task." When they work separately, no comma is necessary. "It's a grand old flag."
A subordinate clause depends on another clause to make sense, and it may begin with "because," "since" or "when." A comma should not separate a subordinate clause that follows the main clause. "The beach party broke up when the police arrived." If a subordinate clause introduces the sentence, a comma must follow it. "When the police arrived, the beach party broke up."
Authorities differ over commas in a series. The AP Stylebook prefers no comma before the conjunction in a simple series. "The American flag is red, white and blue." The final comma may be needed to separate a long or complex series. "You should decide when to have the party, where to have it, and who you want to invite" or to avoid repetition of the conjunction. "He ordered tomato juice, bagels and lox, and coffee for breakfast."