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September 20, 2013

Comma Usage and Compound Predicates

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by David Becker

Time for a brief review of Grammar 101! As you know, a sentence has two major components—the subject and the predicate. The subject is the person, place, or thing that the sentence is about. The predicate says something about the subject. Here is a basic example with the subject in blue and the predicate in red:

Ritija scratched the cat’s head.

A compound predicate says two or more things about the subject. A common mistake people make is to insert a comma between two elements of a compound predicate. This comma adds a pause that creates distance between the subject and the predicate, so you should not include a comma in this case, just as you would not use a comma to separate a verb from its subject or object. Here is an example that demonstrates the correct and incorrect ways to write a sentence with a compound predicate:

Correct: Ritija scratched the cat’s head and rubbed his belly.
Incorrect: Ritija scratched the cat’s head, and rubbed his belly.

However, a compound predicate with three or more elements constitutes a list, so it would be correct to separate them with commas. Commas are essential in this case to distinguish one element of the compound predicate from the rest. (Don’t forget the serial comma!)

The cat closed his eyes, purred, and twitched his ears.

Also note that a sentence with a compound predicate is different from a sentence with two independent clauses joined by a conjunction. The latter requires a comma to create a pause between two distinct thoughts that could be separate sentences. Here’s an example:

Correct: Ritija scratched the cat’s head, and the cat purred.
Incorrect: Ritija scratched the cat’s head and the cat purred.

I hope this will help you avoid incorrect comma usage in the future. For more information on when and when not to use commas, see section 4.03, pages 88–89 of the Publication Manual. If you have any other questions, feel free to contact us or comment on this post.

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