Comma rules
The main function of a comma is to slow the reader down briefly and make the reader pause. A comma is needed when there is a break in thought. In this light, a comma can be misused for two reasons:
using a comma when there is no break in thought, and
omitting a comma when there is a break in thought.
Misusing commas can make the reader confused momentarily or misunderstand a sentence entirely. Based on this essential reason for commas, let's examine comma rules and the common cases of misusing commas. To identify comma rules, we need to know when there is a break in thought. There are five cases.
Independent clauses
There is a break in thought when we combine two independent clauses with coordinating conjunctions such as and, but, or, nor, and for. The comma goes after the first clause and before the coordinating conjunction that separates the clauses.
Incorrect: Plato maintained that what really exists are ideas but Aristotle believed that Plato was wrong.
Correct: Plato maintained that what really exists are ideas, but Aristotle believed that Plato was wrong.
A very short sentence needs not have a comma if the two clauses are closely connected in meaning.
Correct: The gun went off and everyone jumped.
2. Lists
There is a break in thought when we list elements. So, we need a comma to separate elements in a list. The elements of the series must be in a parallel construction, and any of the coordinating conjunctions can be used to connect the last two elements. The series as a whole should not be separated from the rest of the sentence. Grammarians disagree on the use of the final comma in a series, so whether to use the final comma can be guided by clarity of the sentence.
Incorrect: Do you like your steak, rare, medium, or well done?
Correct: Do you like your steak rare, medium, or well done?
Incorrect: You can enter the house, through the front door, through the hole in the wall, and through the open window.
Correct: You can enter the house through the front door, through the hole in the wall, and through the open window.
Incorrect: Plato, Aristotle, and Kant, are famous philosophers.
Correct: Plato, Aristotle, and Kant are famous philosophers.
In a series of adjectives, a comma is used if each adjective separately modifies a noun. If an adjective is so closely linked with the noun that it is thought of as part of a noun, a comma is not used between adjectives. In other words, if you can change the order of the adjectives, put in commas. If you cannot, do not use commas.
Correct: They are inexpensive, delicious, and flavorful vegetables.
Incorrect: They were a large, bath towel and a frilly, party dress.
Correct: They were a large bath towel and a frilly party dress.
3.Transitional words or phrases
A comma is used after transitional words and phrases. These are examples of transitional words and phrases:
however, moreover, of course, on the other hand, and for example.
These words or phrases are called conjunctive adverbs or conjunctive adverbial phrases. They modify the whole sentence. They are set off by commas when they serve to mark a contrast or the introduction of a new point. In short sentences where stress on the transitional word is not needed, commas can be omitted. Also, words or phrases or clauses that interrupt the sentence should be set off by commas.
Correct: Now then, let's get down to work.
Correct: The book, I am happy to say, won the prize.
Correct: We do not know the name of Plato’s son. However, we do know the name of Aristotle’s son.
In the following, “however" is not set off by a comma because it is used as a simple adverb that modifies “much.”
Correct: However much you try, I will not be persuaded.
4. Introductory phrases
Use a comma to separate introductory phrases or clauses. If the introductory phrase is a gerund, participial, or infinitive phrase, use a comma even if the phrase is short. Otherwise the reader may be confused.
Correct: To understand the movie, you have to read the book the movie is based on.
Correct: Having read the book, I was able to understand the movie.
Incorrect: After I eat you can play the music.
Correct: After I eat, you can play the music.
Incorrect: Ever since she has not written a word.
Correct: Ever since, she has not written a word
.
Incorrect: In case you haven’t noticed her cat is missing.
Correct: In case you haven’t noticed, her cat is missing.
5. Nonrestrictive modifiers
Use commas with nonrestrictive (non-identifying) modifiers, but do not use commas with restrictive modifiers. To understand this comma rule, we first need to understand the role of modifiers. A modifying word, phrase, or clause following a noun provides information which can be essential or inessential. A modifier is essential when it identifies what we are talking about. It is inessential when it adds merely more information.
essential = restrictive = identifying --> no comma
inessential = non-restrictive = extra --> yes comma
An essential modifier is also called a restrictive (or identifying) modifier, and an inessential modifier is called a nonrestrictive modifier. When we remove the essential modifier the meaning of the sentence is ruined. Since the modifier is not parenthetical, we do not use commas with restrictive modifiers.
Correct: Anyone with a valid identification card will be admitted.
Correct: Disqualify any students who cheat on the test.
Correct: The water bottle lying on the blue bench belongs to that person.
Correct: Bananas that are green are not ripe.
'With a valid identification card,' 'who cheat on the test,' 'lying on the blue bench' and 'that are green' are modifiers. These modifiers identify which person, students, water bottle and banana we are talking about. If the modifiers are removed, we would not know which. By contrast, a nonrestrictive modifier supplies additional information about something that has already been identified. It adds information that is not essential to our understanding of the sentence. If we remove it from the sentence, we lose some detail, but the overall meaning of the sentence remains the same. To tell the reader that the modifier is nonrestrictive, we use commas.
Care must be taken with a relative clause. If the information in the relative clause is essential to the meaning of the entire sentence, use 'that.' Otherwise, use 'which' and separate it using a comma.In the following, the modifier in the first sentence identifies which dog, but not in the second sentence. 
restrictive vs non-restrictive
Correct: The dog that is not mine follows me around.
Correct: Dogs, which are loyal, follow around their masters.
A beautiful collie, Skip was my favorite dog.
Skip, a beautiful collie, Skip was my favorite dog.
In these sentences, the appositive is "A beautiful collie." Appositives can also be restrictive or nonrestrictive. With the restrictive appositive, we do not place commas around the appositive. With the nonrestrictive appositive, we do place commas around the appositive.
Correct: The word US is an acronym for the United States.
Correct: Our friend Susan needs our help.
“US” and "Susan” are appositives. They are essential to understand the meaning of the whole sentences. “Word” and “friend” are unspecific. We need to know ‘which word’ and ‘which friend’ to understand the sentences. If a sentence would be clear and complete without the appositive, commas should be placed around the appositive.
Correct: New York, a city that never sleeps, is worth visiting.
Correct: Dogs, the best friends of men, are loyal.
Correct: Susan, our friend, needs our help.
In these cases, commas are needed since the meaning is intact without the appositives.
Context-dependency
In many cases, whether a modifier is used as restrictive or nonrestrictive depends on the context. If the listener already knows whom or what I am talking about, commas around the modifiers should be used. If the listener does not know which or what, and a modifier is used to identify which or what, commas should not be used around the modifier. So it is possible that, in exactly the same sentences, commas can be required or must be removed depending on the context. Consider the following sentences:
My dog, whose name is Tiger, is a picky eater.
My dog whose name is Tiger is a picky eater.
The first sentence means, “I have a picky eating dog and by the way his name is Tiger.” The context here is that the lister knows that I have just one dog or knows which dog I am talking about. The speaker simply reminds the listener of its name. The second sentence means ‘among my many dogs, the dog named Tiger is a picky eater.' The omission of commas implies that I have more than one dog, and the name identifies the dog that I’m talking about. Without the name, the listener will be unsure of which one I am talking about.
If a relative clause is used as a restrictive modifier, 'that' is often used without a comma. If the relative clause is used as a nonrestrictive modifier, 'which' is used with a comma.
She gave me the notebook, which has a black cover.
She gave me the notebook that has a black cover.
In the first sentence, it is known by the context that there was only one notebook, and it happend to have a black cover. In the second sentence, it is implied that there are multiple notebooks of different colors, and she gave me the book with black cover.
Wrong comma use
One of the most common writing mistakes in students’ writing is to add commas when they should not be there. Indeed, using commas where they do not belong is a far more serious error than omitting them where they might be expected. Not using them when needed can be viewed as carelessness, but using them in the wrong place is clear evidence of ignorance.
Commas should never be used to separate a subject and a predicate, or a predicate and its complement.
Incorrect: The old car that has been abandoned in the park, is an eyesore.
Correct: The old car that has been abandoned in the park is an eyesore.
Incorrect: She knew immediately, what was going to happen next.
Correct: She knew immediately what was going to happen next.
Do not use a comma before a coordinating conjunction (and, but, for, nor, or, so, yet) when the conjunction does not join parts of a compound sentence. In the following examples, no comma should be used.
Incorrect: This practice can be seen in suburbs, and in downtowns.
Correct: This practice can be seen in suburbs and in downtowns.
Incorrect: My mother, and her sisters went to see a performance.
Correct: My mother and her sisters went to see a performance.
Do not use commas to separate idiomatic phrases.
Incorrect: The book was so boring, that I was happy to give it away.
Correct: The book was so boring that I was happy to give it away.