Commas aid the reader by separating the main part of a sentence from a clause or phrase that adds extra information.
Introductory Clauses and Phrases
A clause is a group of words with a subject and a verb. A phrase lacks one or the other. Either can be used to start a sentence:
- While John was studying for his English exam, he felt the urge to go for a walk.
- While studying for his English exam, John felt the urge to go for a walk.
The main part of the sentence is he felt the urge to go for a walk. The introductory element merely gives us a reason or context for John’s feeling.
Some students question the need for the comma, yet its function becomes clear when we remove it from the following sentence:
- Although John liked Mary Nelson liked her better.
Without the aid of a comma, the reader stumbles over the sentence before realizing that “Mary Nelson” is not one person.
Other Introductory Elements
Offset short one- or two-word introductions:
- Naturally, Susan wanted to go out and play.
- Brandon hated the film. For example, he said the plot was too unrealistic.
Some short introductory words do not require a comma when the meaning is already clear:
- Now you can find almost anything online.
- Soon it will all be over.
Trailing Clauses and Phrases
Trailing clauses and phrases also provide additional information but they come at the end of the sentence:
- Robert Mitchum starred in The Night of the Hunter, a film directed by Charles Laughton.
- It rained, causing the party to be cancelled.
Novice writers may be confused when a trailing or introductory phrase is longer than the main part of the sentence, as in the second example, above. The deciding factor, however, is that the first clause can stand on its own as a complete sentence; the second cannot.
Introductory elements can also be moved to the end of a sentence or even in the middle. As a general rule, include a comma if it clarifies the meaning of the sentence or expresses a break in thought. Note that commas are needed in the following sentences
- Susan wanted to go out and play, naturally.
- He said the plot, for example, was too unrealistic.
but not in this one:
- John felt the urge to go for a walk while studying for his English exam.
What If Your Sentence Contains Both an Introductory and a Trailing Element?
In general, writers should avoid creating sentences where commas are needed to offset both introductory and trailing thoughts:
- While waiting for the storm to pass, Martin told the children ghost stories, frightening Annie and Josh.
In this case, the main part of the sentence (Martin told the children ghost stories) becomes obscured by the two additional elements. The reader naturally expects the sentence to end after “stories.”
Writers can avoid such confusion in a number of ways, such as using a dash
- While waiting for the storm to pass, Martin told the children ghost stories— frightening Annie and Josh.
or by splitting the sentence in two:
- While waiting for the storm to pass, Martin told the children ghost stories. Annie and Josh were frightened by his eerie tales.
Correct the following sentence? (Answer appears at the end of the article.)
- Fortunately Martha booked the recital hall allowing us to hold the party the next day.
There are many other uses for commas, but using them to join sentences (Rule No. 1), offset interrupters (Rule No. 2), and separate introductory and trailing ideas will give your writing variety and spice and in a clear, easy-to-read fashion. (One answer: “Fortunately, Martha booked the recital hall. Her resourcefulness allowed us to hold the party the next day.”)