by Davy Kraken
A library featuring commonly committed errors of the English language.
|As a general rule, a comma should only appear before a coordinating conjunction for two reasons:
It joins two independent clauses.
It acts as a serial comma; that is, the optional comma that precedes the conjunction before the last item in a series of three or more items, such as before “and” in the phrase “me, myself, and I.”
(For more information on coordinating conjunctions and clauses, see this entry: "Clauses" .)
When a coordinating conjunction is simply used to join two equal parts within a clause, usually no comma should separate them. This applies to any part of speech, but the instance in which the rule is broken most often and therefore deserves special mention is when we have a double predicate.
Double predicate is a fancy term for when two verbs apply to the same subject such as in the following sentence:
We ate popcorn and drank a Coke.
“Ate” and “drank” both describe the actions of “we.” The following is another way to say the same thing:
We ate popcorn, and we drank a Coke.
If we added another verb to the first sentence, we’d have the triple predicate below:
We ate popcorn, drank a Coke, and watched the movie.
The first sentence has no comma because double predicates shouldn’t contain a comma just like a double subject wouldn’t contain a comma:
Erica and I ate popcorn and drank a Coke.
Why, then, does the second sentence have a comma? Because it’s not a double predicate. Though “we” performs both actions, each “we” is, in grammatical terms, a separate subject, so there are two single predicates. A double predicate occurs only when the verbs apply to the exact same word in the sentence.
Whenever at least a triple predicate exists, as in the third sentence, commas should be used just as they would be used between three or more subjects applying to the same verb(s):
Erica, Stacy, and I ate popcorn and drank a Coke.
Jacob walked in the door, and he took a hot bath.
Jacob walked in the door, took a hot bath, and went to bed.
Jacob walked in the door, shivering from the cold, and took a hot bath.
This is correct comma usage because the rules about separating the participial phrase “shivering from the cold” override the rules concerning double predicates. (For more information on participial phrases, see this entry: "Participial Phrases" .)
I’ll go to the supermarket to get dinner and fill up the car with gas.
However, constructing the sentence this way might cause confusion. This is meant to communicate, “I’ll go to the supermarket to get dinner,” and, “I’ll fill up the car with gas,” but without the comma, it could be read as “I’ll go to the supermarket to get dinner” and “I’ll go to the supermarket to fill up the car with gas.” The latter doesn’t make any sense, but it’s better not to make your reader stop and reason through that. In cases where confusion may result from no comma usage, many consider it acceptable to use a comma:
I’ll go to the supermarket to get dinner, and fill up the car with gas.
However, another possible remedy is to rearrange the predicates:
I’ll fill up the car with gas and go to the supermarket to get dinner.
If the order of the errands is important, then adding “then” to the original sentence would do the charm:
I’ll go to the supermarket to get dinner and then fill up the car with gas.
Or, as a final option, restate the subject and avoid the double predicate altogether:
I’ll go to the supermarket to get dinner, and (then) I’ll fill up the car with gas.