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 4 cases adds clarity to your writing, but using commas where they're not needed will cause confusion.
Read on to learn how to use commas properly.
1) Serial (Oxford) comma
Using commas in a series tells your reader that you are providing a list. Your reader may be confused otherwise.
For example:
Her house was decorated in shades of green, blue, and purple. (a list of 3 colors)
We elect official representatives: a president, vice-president, secretary and treasurer. (How many were elected? 3 or 4?)
2) Coordinating Conjunction
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.
3) Introductory comma
Using commas after introductory words, phrases, and clauses tells your reader what is the 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 icy 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
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 beach I went to the library to study.
Right – Even though I would rather go to the beach, I went to the library to study.
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.
4) Comma Use
Non-essential elements may be important description, but they are not essential to the construction of a sentence. 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.)
Commas used to set off non-essential elements indicate which parts of the sentence are extra description or an aside comment.
For example:
Tropicana Fields, currently located in St. Petersburg, may be moved north to Clearwater.
The students, hoping to get the best classes, enrolled before Fall semester.
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 St. Petersburg Times, which has great articles on ecological issues. (Non-essential elements can come at the end of a sentence.)
5) Other uses of commas provide clarity in details: