1. Use a comma to separate the elements in a series (three or more things), including the last two. "He hit the ball, dropped the bat, and ran to first base." You may have learned that the comma before the "and" is unnecessary, which is fine if you're in control of things. However, there are situations in which, if you don't use this comma (especially when the list is complex or lengthy), these last two items in the list will try to glom together (like macaroni and cheese). Using a comma between all the items in a series, including the last two, avoids this problem. This last comma—the one between the word "and" and the preceding word—is often called the serial comma or the Oxford comma. In newspaper writing, incidentally, you will seldom find a serial comma, but that is not necessarily a sign that it should be omitted in academic prose.
2. 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."
Contending that the coordinating conjunction is adequate separation, some writers will leave out the comma in a sentence with short, balanced independent clauses (such as we see in the example just given). If there is ever any doubt, however, use the comma, as it is always correct in this situation.
One of the most frequent errors in comma usage is the placement of a comma after a coordinating conjunction. We cannot say that the comma will always come before the conjunction and never after, but it would be a rare event, indeed, that we need to follow a coordinating conjunction with a comma. When speaking, we do sometimes pause after the little conjunction, but there is seldom a good reason to put a comma there.
3. Use a comma to set off introductory elements, as in "Running toward third base, he suddenly realized how stupid he looked."
It is permissible to omit the comma after a brief introductory element if the omission does not result in confusion or hesitancy in reading. If there is ever any doubt, use the comma, as it is always correct.
4. Use a comma to set off parenthetical elements, as in "The Founders Bridge, which spans the Connecticut River, is falling down." By "parenthetical element," we mean a part of a sentence that can be removed without changing the essential meaning of that sentence. The parenthetical element is sometimes called "added information." This is the most difficult rule in punctuation because it is sometimes unclear what is "added" or "parenthetical" and what is essential to the meaning of a sentence.
Appositives are almost always treated as parenthetical elements.
* Calhoun's ambition, to become a goalie in professional soccer, is within his reach.
* Eleanor, his wife of thirty years, suddenly decided to open her own business.
Sometimes the appositive and the word it identifies are so closely related that the comma can be omitted, as in "His wife Eleanor suddenly decided to open her own business." We could argue that the name "Eleanor" is not essential to the meaning of the sentence (assuming he has only one wife), and that would suggest that we can put commas both before and after the name (and that would, indeed, be correct). But "his wife" and "Eleanor" are so close that we can regard the entire phrase as one unit and leave out the commas. With the phrase turned around, however, we have a more definite parenthetical element and the commas are necessary: "Eleanor, his wife, suddenly decided to open her own business." Consider, also, the difference between "College President Ira Rubenzahl voted to rescind the withdrawal policy" (in which we need the name "Ira Rubenzahl" or the sentence doesn't make sense) and "Ira Rubenzahl, the college president, voted to rescind the withdrawal policy" (in which the sentence makes sense without his title, the appositive, and we treat the appositive as a parenthetical element, with a pair of commas).
When a parenthetical element — an interjection, adverbial modifier, or even an adverbial clause — follows a coordinating conjunction used to connect two independent clauses, we do notput a comma in front of the parenthetical element.
* The Red Sox were leading the league at the end of May, but of course, they always do well in the spring. [no comma after "but"]
* The Yankees didn't do so well in the early going, but frankly, everyone expects them to win the season. [no comma after "but"]
* The Tigers spent much of the season at the bottom of the league, and even though they picked up several promising rookies, they expect to be there again next year. [no comma after "and"]
When both a city's name and that city's state or country's name are mentioned together, the state or country's name is treated as a parenthetical element.
* We visited Hartford, Connecticut, last summer.
* Paris, France, is sometimes called "The City of Lights."
When the state becomes a possessive form, this rule is no longer followed:
* Hartford, Connecticut's investment in the insurance industry is well known.
Also, when the state or country's name becomes part of a compound structure, the second comma is dropped:
* Heublein, a Hartford, Connecticut-based company, is moving to another state.
An absolute phrase is always treated as a parenthetical element, as is an interjection. An addressed person's name is also always parenthetical. Be sure, however, that the name is that of someone actually being spoken to. A separate section on Vocatives, the various forms that a parenthetical element related to an addressed person's name can take, is also available.
*Their years of training now forgotten, the soldiers broke ranks.
*Yes, it is always a matter, of course, of preparation and attitude.
*I'm telling you, Juanita, I couldn't be more surprised. (I told Juanita I couldn't be more surprised. [no commas])
5. Use a comma to separate coordinate adjectives. You could think of this as "That tall, distinguished, good looking fellow" rule (as opposed to "the little old lady"). If you can put an and or a but between the adjectives, a comma will probably belong there. For instance, you could say, "He is a tall and distinguished fellow" or "I live in a very old and run-down house." So you would write, "He is a tall, distinguished man" and "I live in a very old, run-down house." But you would probably not say, "She is a little and old lady," or "I live in a little and purple house," so commas would not appear between little and old or between little and purple.
6. Use a comma to set off quoted elements. Because we don't use quoted material all the time, even when writing, this is probably the most difficult rule to remember in comma usage. It is a good idea to find a page from an article that uses several quotations, photocopy that page, and keep it in front of you as a model when you're writing. Generally, use a comma to separate quoted material from the rest of the sentence that explains or introduces the quotation:
* 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."
If an attribution of a quoted element comes in the middle of the quotation, two commas will be required. But be careful not to create a comma splice in so doing.
*"The question is," said Alice, "whether you can make words mean so many things."
* "I should like to buy an egg, please," she said timidly. "How do you sell them?"
Be careful not to use commas to set off quoted elements introduced by the word that or quoted elements that are embedded in a larger structure:
* Peter Coveney writes that "[t]he purpose and strength of . . ."
* We often say "Sorry" when we don't really mean it.
And, instead of a comma, use a colon to set off explanatory or introductory language from a quoted element that is either very formal or long (especially if it's longer than one sentence):
* Peter Coveney had this to say about the nineteenth-century's use of children in fiction: "The purpose and strength of . . . . "
7. Use commas to set off phrases that express contrast.
*Some say the world will end in ice, not fire.
*It was her money, not her charm or personality, that first attracted him.
*The puppies were cute, but very messy.
(Some writers will leave out the comma that sets off a contrasting phrase beginning with but.)
8. Use a comma to avoid confusion. This is often a matter of consistently applying rule #3.
*For most the year is already finished. *For most, the year is already finished.
*Outside the lawn was cluttered with hundreds of broken branches.
*Outside, the lawn was cluttered with hundreds of broken branches.
9. Grammar English's Famous Rule of Punctuation: Never use only one comma between a subject and its verb. "Believing completely and positively in oneself is essential for success." [Although readers might pause after the word "oneself," there is no reason to put a comma there.]
10. Typographical Reasons: Between a city and a state [Hartford, Connecticut], a date and the year [June 15, 1997], a name and a title when the title comes after the name [Bob Downey, Professor of English], in long numbers [5,456,783 and $14,682], etc. Although you will often see a comma between a name and suffix — Bob Downey, Jr., Richard Harrison, III — this comma is no longer regarded as necessary by most copy editors, and some individuals — such as Martin Luther King Jr. — never used a comma there at all.
11. Use Commas With Caution
As you can see, there are many reasons for using commas, and we haven't listed them all. Yet the biggest problem that most students have with commas is their overuse. Some essays look as though the student loaded a shotgun with commas and blasted away. Remember, too, that a pause in reading is not always a reliable reason to use a comma. Try not to use a comma unless you can apply a specific rule from this page to do so.
Concentrating on the proper use of commas is not mere form for form's sake. Indeed, it causes writers to review their understanding of structure and to consider carefully how their sentences are crafted.
Credit : GrammarGuide