Comma? No Comma?

Commas set things apart. If you are unsure about needing a comma, ask yourself, “What boundary am I marking here? What am I separating?”

To understand “comma rules” you must be able to identify independent (main) and dependent (subordinate) clauses and to distinguish restrictive (identifying) from nonrestrictive (parenthetical) elements in your own sentences.

Use a comma to separate elements in a series

                 We cooked hot dogs, hamburgers, and chicken on the grill. [Note: There is a comma before “and.”]

and to mark the end of the first independent clause in a compound sentence where two independent clauses are joined by a coordinating conjunction (and, but, or, for, nor, so, yet),

           It rained hard, but they kept playing.

           The counselors cooked the hamburgers, and the campers roasted marshmallows.

but not in a compound verb

We cooked hamburgers and roasted marshmallows.

[You don’t want to separate the subject from part of its verb.]

*Commas help readers separate an introductory element from the element being introduced.

Use a comma after introductory phrases

         In almost every case, the study found a significant conflict of interest.

but it’s optional after very short introductory phrases

               In 2007 Congress debated immigration reform.

Use a comma after an introductory dependent clause

         Because wholesale prices were rising, our costs were going up.

but not before most dependent clauses (because the subordinating conjunction* already signals “New clause coming”)

      Our costs were going up because wholesale prices were rising.

*Some other common subordinating conjunctions are when, since, unless, if, before, after, while, until, whether.

 Exception: When the independent clause is followed by a subordinate clause introduced by “although/even though,” a comma   does sometimes set off the subordinate clause when it has a reflective “afterthought” quality: 

I didn’t quit, although I thought about it.

When a month/day/year date comes in the middle of a sentence, a comma follows the year

Edith Wharton was born on January 24, 1862, in New York.

When a city and state come in the middle of a sentence, a comma follows the state

The College was founded in Lynchburg, Virginia, in 1891 and opened in 1893.

Use commas like a set of parentheses to set off parenthetical (nonrestrictive) phrases or clauses

In The Woman Warrior, a highly acclaimed autobiographical work, Maxine Hong Kingston writes about growing up in two    cultures.

Professor Jones, who is conducting the workshop, is an expert on environmental law.

but don’t separate restrictive (essential, identifying) phrases or clauses from the word(s) they modify

The highly acclaimed autobiographical work The Woman Warrior was Maxine Hong Kingston’s first book.

The woman who is conducting the workshop is an expert on environmental law.