A comma is mainly used to separate sentence elements in order to help the reader understand the meaning of the sentence more clearly.

 
 

Introductory Elements

Use a comma after an introductory word, phrase, or clause.

Incorrect: First we have to finish building the house.

Correct: First, we have to finish building the house.

Incorrect: In fact the doctor did say that the prognosis was positive.

Correct: In fact, the doctor did say that the prognosis was positive.

Incorrect: While getting ready for the party the girls quizzed each other on grammatical rules.

Correct: While getting ready for the party, the girls quizzed each other on grammatical rules.

 

Compound Sentences

A compound sentence is made up of two or more parts that could stand alone as a sentence. If the parts are joined by a coordinating conjunction (and, but, for, nor, or, so, yet), use a comma before the conjunction.

Incorrect: He said he was ready to take the exam but the teacher knew that he needed more time to study.

Correct: He said he was ready to take the exam, but the teacher knew that he needed more time to study.

Please Note:

When a coordinating conjunction joins an independent clause with a dependent clause, a comma is unnecessary.

Incorrect: Sherry decided she wanted to go, but knew she had no money for the trip.

Correct: Sherry decided she wanted to go but knew she had no money for the trip.

 
 

Items in a Series

When three or more items appear in a series, they should be separated from one another with commas.

Incorrect: Jennifer installed a new modem a CD-ROM and a hard drive on her old computer.

Correct: Jennifer installed a new modem, a CD-ROM, and a hard drive on her old computer.

 
 

Nonrestrictive Elements

Use commas to set off a nonrestrictive element, a part of a sentence that tells more about a word in a sentence but that your reader does not need in order to understand the word or sentence.

Incorrect: Mr. Allen a 75-year-old doctor was the guest speaker at the gerontology conference.

Correct: Mr. Allen, a 75-year-old doctor, was the guest speaker at the gerontology conference.

 
 

Restrictive Elements

A restrictive element is one that is essential to the basic meaning of the sentence. It is not set off from the rest of the sentence with a comma or commas.

Incorrect: Researchers, who want to test an experimental medication, must get consent from the FDA.

Correct: Researchers who want to test an experimental medication must get consent from the FDA.

Incorrect: Shakespeare's play, Macbeth, portrays the negative side of human nature.

Correct: Shakespeare's play Macbeth portrays the negative side of human nature. (Since Macbeth is not the only play that Shakespeare wrote, it should not be set off with commas.)

 
 

Comma Quiz

 
 

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