College Prep Writing
A misplaced comma is one of the most common errors in written English. Some reasons for the errors are that the use of a comma is sometimes optional, a semicolon can sometimes replace a comma, use of commas differ in American and British English, and, finally, style manuals do not always agree on certain uses of commas. As a result, writers are often confused about correct comma usage. Others feel that the comma is so small, a fraction of the size of the smallest letter, that no one will notice the misplacement of a comma. Unfortunately, a misplaced comma can seriously affect meaning in written English. (See also "Punctuation Points" in Fixing Writing Errors, and "The Style Manual" in Essays, College Prep Writing.)
Commas With Independent Clauses
Use a comma before a coordinating conjunction when the conjunction connects two independent clauses. Coordinating conjunctions include and, but, or, so, yet, and nor. For is an infrequently used coordinating conjunction.
I practice a lot, but I don't seem to get better.
I thought the test would be easy, so I did not bother to study.
I can buy a new tablet computer, or I can rent one for the semester.
Coordinating conjunctions do not always connect clauses. The conjunctions in the following sentences are not preceded by commas.
He was late but not too late.
Martin makes ice cream and bakes bread.
Ask Jorge or Maria.
Commas With Dependent Clauses
Dependent clauses are introduced with subordinating conjunctions. Examples are because, if, when, while, where, since, unless, although, after, and before.
Use a comma after a dependent clause when the dependent clause is followed by an independent clause.
Since you got your new job, you don't call me anymore.
Although we left early, we arrived late.
Unless you keep trying, you will not succeed.
A comma is not needed when the independent clause comes before the dependent clause.
I need to get your car keys before you leave.
Let's watch TV while we are waiting.
Commas With Lists
Three or more items in a series in a sentence are separated by commas; a comma also separates the last item in the series from the one preceding it.
My career experience includes managing a hospital, running an airline, and supervising teachers.
We have students from Jordan, Peru, Chad, and Mali in our class.
A comma does not separate two items in a series.
I am going to invite Barry and Kyle to my party.
Commas With Noun Modifiers
Separate two or more adjectives that precede a noun when and could be used naturally instead of a comma.
a long and winding road • a long, winding road
a hot and humid day • a hot, humid day
When an adjective and a noun make a logical pair, the pair is not separated by a comma from a single adjective that precedes the pair.
typical wintry weather • spicy Thai food
But: delicious, spicy Thai food
Commas With Appositives
An appositive is a modifier—a word, a phrase, or a clause that gives additional information about a noun or a phrase in a sentence. A nonrestrictive appositive is extra, unessential information, and is set off with commas. A restrictive appositive provides information that is essential to an understanding of the complete meaning of a sentence.
Anna and Ken Sato are unessential information in these examples.
Nonrestrictive: Hanna's sister, Anna, is an architect.
The commas inform the reader that Hanna has only one sister.
Nonrestrictive: My best friend, Ken Sato, is president of a software company.
The following sentence includes a nonrestrictive clause. It tells the reader that the writer has only one uncle. Separate a nonrestrictive clause with commas from the rest of the sentence.
Nonrestrictive: My uncle, who is ninety-three, still jogs a mile every morning.
A restrictive appositive is information that is essential to the meaning of a sentence. With restrictive appositives, commas are not necessary.
Restrictive: Hanna's brother George is also an architect.
Restrictive: Carolyn gave her sister Sally a self-portrait.
No commas separate the restrictive appositives George and Sally, and that tells the reader that George and Sally are essential to the meaning of the sentences. The lack of commas tells the reader that Hanna has more than one brother and Carolyn has more than one sister.
In the example that follows, the restrictive clause, who is ninety-one, provides necessary information. The information is essential because it distinguishes one aunt from others.
Restrictive: My aunt who is ninety-one is very fit for her age.
Commas With Introductory Phrases
The use of a comma with an introductory phrase requires good judgment.
Use a comma when it will make reading easier. In general, a comma after a lengthy introductory phrase will help a reader. However, there may be occasions when length is not the determining factor for use of a comma; such is the case in the following example.
In the beginning, everyone thought the idea for the Silicon Valley start-up was half-baked.
The brief two-word adverb that introduces the sentence below does not need a comma, but inserting a comma after noon would not contribute to any confusion.
By noon everyone had left.
In general use a comma after adverbs of chronology: first, next, and so forth.
First of all, you need to find a large table to work on.