Five Most Common Comma Errors
Use a comma before a coordinating conjunction (and, but, or, so, nor, for, & yet) when the conjunction is joining two independent clauses (complete sentences)
"Bob decided to go to the movies" is a sentence. "He didn't know what film to see" is also a sentence. Logically, these sentences could be joined with the conjunction "but." When you join two sentences with a coordinating conjunction, place a comma before the conjunction:
Bob decided to go to the movies, but he didn't know what film to see.
Note: Conjunctions can also be used to join two things that are not complete sentences; in these cases, no comma is needed:
Mary and Steve are in the same math class.
I went bowling and rolled a perfect game.
Use a comma after an introductory phrase or clause (group of words)
Frequently, writers begin sentences with clauses or phrases before the subject and verb are introduced. These introductions often provide more information about where, when, why, or how the action of the sentence happened. Place a comma after such an introductory phrase or clause.
Because my left leg was in a cast, I asked Maggie to drive the car.
When Roger heard his name announced at the graduation ceremony, he sprang to his feet.
Underneath the Johnson's front porch, the three boys plotted their scheme.
Surround transitional words and phrases with commas
Transitional words and phrases should be surrounded with commas regardless of where they occur in the sentence.
However, the Brewers lost the World Series in seven games.
The Brewers, however, lost the World Series in seven games.
The Brewers lost the World Series in seven games, however.
The following are some common transitional words and phrases that are almost always surrounded by commas: additionally, as a result, furthermore, in addition, moreover, in the same way, likewise, similarly, however, nevertheless, in contrast, first, second, third, finally, meanwhile, for example, for instance, indeed, of course, on the other hand, consequently, hence, therefore, thus.
Note: If you use a transition word to join two sentences, you must use a semicolon before it and a comma after it.
Bob decided to go to the movies; however, he didn't know what film to see.
Set off nonessential elements with commas
Sometimes a writer may supply extra information about a noun (person, place, or thing) that is NOT essential to the sentence; in other words, you would understand what person, place, or thing the writer is referring to without the extra information. Set off such extra information with commas.
Ernest Hemingway, author of For Whom the Bell Tolls, took his own life in 1961.
My new jacket, which my mother gave me for my birthday last month, came back from the cleaners with a large rip.
Mary is very happy with her new car, a Volkswagen Beetle.
In each of these cases, the sentence would retain its core meaning even if the stuff set off by commas was removed.
Note: Clauses that start with the word "which" are frequently (but not always) nonessential and should be surrounded by commas. Clauses that start with the word "that" are always essential clauses and should never be surrounded by commas. Example:
The tree that I just planted was run over by the lawnmower.
The small spruce tree, which I just planted, was run over by the lawnmower.
Use commas to separate items in a list or series
John, Mary, Bob, and Sue are graduating this semester.
I drove to the beach, parked my car, and stared at the waves for an hour.
Note: Although some experts say that using a comma before the "and" at the end of a list is optional, most style books recommend using one because it can help avoid confusion in some instances.
Always have a reason for every comma you use. It is also a good idea to figure out which comma rule(s) you violate the most and proofread carefully for it/them before turning in your essay.