Commas separate parts of a sentence or set off words or phrases from the rest of a sentence.
There are 4 main uses of commas in sentence construction. Using a comma in these four cases adds clarity to your writing, but using commas where they're not needed will cause confusion.
Comma Use One -
Using commas in a series tells your reader that you are providing a list. Your reader may be confused otherwise. For example:
Wrong - We elect official representatives: a president, vice-president, secretary and treasurer. (How many were elected? 3 or 4?)
Right - Her house was decorated in shades of green, blue, and purple. (a list of 3 colors)
Comma Use Two -
Using commas with coordinating conjunctions tells your reader that you are discussing two related sentences, not just two things.
When combining sentences into a compound sentence, you need a comma before the coordinating conjunction. For example:
I like peanut butter, and I like jelly.
He eats macaroni, but he won't eat cheese.
BUT when combining two nouns or verbs, you don't need a comma. For example:
I like peanut butter and jelly.
He eats macaroni or cheese but not both.
Comma Use Three –
Using commas after introductory words, phrases, and clauses tells your reader what is introduction and what is the main point. For example:
In the back of the room, two kids whispered and giggled. (Two kids whispered is the main point; in the back just describes where.)
Slipping and sliding, the car inches slowly across the wet roads. (The car inches is the main point; slipping just describes how.)
NOTE this special kind of introductory element - the dependent clause of a complex sentence.
1) If the subordinating conjunction comes at the beginning of the sentence. a comma comes at the end of the dependent clause.
Wrong – Even though I would rather go to the lake I went to the library to study.
Right – Even though I would rather go to the lake, I went to the library to study.
2) If the subordinating conjunction comes in the middle or at the end of the sentence – no comma is required.
Wrong – School is not all about studying, since there are lots of clubs and fun activities on campus.
Right – School is not all about studying since there are lots of clubs and fun activities on campus.
Comma Use Four –
Non-essential elements may be important description, but they are not essential to the construction of a sentence. Commas are used to set off non-essential elements and indicate which parts of the sentence are extra description. If you can remove the element without creating a question, then it is non-essential.
Essential element - Students who play sports must have a good GPA. (Which students must have a good GPA?)
Non-essential element - The teacher, who plays flute, enjoys music. (Any teacher enjoys music, not just flute players.)
The students, hoping to get the best classes, enrolled before Fall semester. (All students enroll - not just those hoping for best classes.)
Each package, however, contained a broken vase. (However is sometimes used to join sentences but can also act as an aside. Note the comma both before and after.)
I subscribe to the Pine Log, which has great articles on ecological issues. (Non-essential elements can come at the end of a sentence.)
Other uses of commas provide clarity in details:
1) Use commas to separate two or more adjectives:
The girl was very fond of her little, fluffy, black kitties.
2) Use commas in dates, addresses, place name, and long numbers:
1600 Pennsylvania Ave., Washington, D.C., is where the White House is located.
July 4, 1776 is when the Declaration of Independence was signed.
I won 1,000,000 dollars in the Texas Lottery!
3) Use commas to set off a direct quotation from the rest of the sentence:
My mother used to tell me, "What goes around comes around."
"Come live with me and be my love," quoted the romantic young lumberjack.