Use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.
"He was merely ignorant, not stupid."
"The chimpanzee seemed reflective, almost human."
Appositives and appositive phrases - use commas to set off and enclose an appositive (a word or phrase which can be substituted for a name - do not confuse this rule for renaming a noun with merely describing a noun.)
Example: "Tony Ahern, the captain of the soccer team, is in my English class."
Note: Short or one word appositives are not set off with commas such as "my friend Bill" or "my sister Maresa."
Interrupters - Use commas to set off introductory words and expressions which interrupt the sentence. These expressions are often called parenthetical expressions because the words themselves are not essential to the sentence and could be placed in parentheses.
Examples of introductory words and interrupters: "yes", "no", "well", "indeed", "nevertheless", "however", "I believe", "in fact", "of course", "in my opinion", "on the other hand", "to tell the truth" and "on the contrary."
When a date or address with several parts occurs in a sentence, place a comma between each element and after the last part.
Place a comma between three or more items connected by a coordinating conjunction.
Use a comma to set off quoted elements.
"Summing up this argument, Peter Coveney writes, 'The purpose and strength of the romantic image of the child had been above all to establish a relation between childhood and adult consciousness.'"
Use commas before or surrounding the name or title of a person directly addressed.
"Will you, Aisha, do that assignment for me?"
Use a comma to separate two adjectives when the word and can be inserted between them.
"He is a strong, healthy man."
"We stayed at an expensive summer resort." You would not say expensive and summer resort, so no comma.
3. Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence. Use one comma before to indicate the beginning of the pause and one at the end to indicate the end of the pause.
Here are some clues to help you decide whether the sentence element is essential:
If you leave out the clause, phrase, or word, does the sentence still make sense?
Does the clause, phrase, or word interrupt the flow of words in the original sentence?
If you move the element to a different position in the sentence, does the sentence still make sense?
Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.
a. Common starter words for introductory clauses that should be followed by a comma include "after", "although", "as", "because", "if", "since", "when" and "while". However, don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).
b. Common introductory phrases that should be followed by a comma include participial and infinitive phrases, absolute phrases, nonessential appositive phrases, and long prepositional phrases (over four words).
c. Common introductory words that should be followed by a comma include "yes", "however", "well".
Use a comma + a little conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses, as in "He hit the ball well, but he ran toward third base."