Commas account for the majority of punctuation errors. Learning the dos and don’ts of comma usage will.

Commas

Common mistakes include leaving out necessary commas, inserting unnecessary commas, and joining two sentences with a comma (called a comma splice). The Purdue University Online Writing Lab outlines related rules to avoid these comma usage errors [1]:

Comma dos:

Use commas between independent clauses that are “joined by…and, but, for, or, nor, so, yet”:
– The presentation was interesting, but I still couldn’t stay awake.
– I urgently emailed my supervisor, yet she hasn’t responded.

Use commas following “introductory clauses, phrases, or words that come before the main clause”:
– While I was evaluating an intern’s performance, Jeff knocked on my door.
– To get a seat, you should arrive early.
– So, the answer is that it depends on the frequency.

Use commas before clauses, phrases, or words that occur at the end of a sentence and modify the beginning or middle of that sentence:
– Karl spoke with his supervisor, humbly admitting his mistake.
– Susan consulted her mentor, who advised her to take the job.

Use commas to “set off clauses, phrases, and words that are not essential to the meaning of a sentence”:
– This Monday, which happens to be the first day of fall, I am calling in sick.
– This will take me about an hour to finish. That, on the other hand, will take a whole day.
– I think, however, you’ll want to revisit that conclusion.

Use commas between words, phrases, or clauses that appear in a list of three or more:
– I have conceptualized, planned, and scheduled my next project.
– To install this software, you’ll need a CD-ROM drive, the installation CD, and the installation code.

Use commas between related adjectives that precede a noun:
– The busy, driven engineer rarely makes mistakes.
– We have struggled with this complex, multifaceted problem for months.

Use commas “to set off geographical names, items in dates, addresses, and titles in names”:
– Is the conference in Las Vegas, Nevada, again this year?
– The last time I looked for a job was December 15, 2005. (but) The last time I looked for a job was

December 2005.
– Are the headquarters still at 1234 Main Street, Washington, DC?
– Phil included Jenny Wilson, PhD, in the last staff meeting.

Use a comma before a quotation:
– Mary said, “I’ll see you tomorrow.”
– According to Nikola Tesla, “The alternating current is the preferred method of distribution.”

Comma don’ts:

Don’t use commas to “set off” words, phrases, and clauses that affect the meaning of a sentence:
Incorrect — Engineers, who don’t write well, may encounter career obstacles.
Correct – Engineers who don’t write well may encounter career obstacles.

Don’t use commas between unrelated adjectives that precede a noun:
Incorrect — A blank, computer screen can be intimidating.
Correct — A blank computer screen can be intimidating.

Don’t use commas “between…verbs…in a compound predicate”:
Incorrect — She turned on her computer, and got straight to work.
Correct — She turned on her computer and got straight to work.

Don’t use commas to separate main clauses and dependent clauses that follow them unless the contrast is significant:
Incorrect — The current was responsive, while I tested it.
Correct — The current was responsive while I tested it.

Incorrect — The current was responsive although slow.
Correct — The current was responsive, although slow.

Don’t use commas to separate two independent clauses (this is a comma splice):
Incorrect — He spends his mornings responding to emails, he writes reports after lunch.
Correct — He spends his mornings responding to emails, then he writes reports after lunch. [or] He spends his mornings responding to emails. He writes reports after lunch.

References

[1] D. Driscoll and A. Brizee. (2012, July 29). Extended rules for using commas.  Purdue Online Writing Lab. [Online]. Available: http://owl.english.purdue.edu/owl/resource/607/02