John ate a slice of pepperoni pizza, and drank a bottle of beer.
If that sentence doesn’t make you involuntarily scowl, then you do not have a proper understanding of commas. Yet sadly, in my experience as an editor as well as a reader of emails and articles from various sources, many educated people do not understand what makes it grammatically indefensible. Keep your eye out and you’ll see it all the time, too.
Let’s break it down.
The rule, per Purdue OWL, is that you should not put a comma between the two verbs or verb phrases in a compound predicate.
“John” is the subject of the sentence. “Ate a slice” and “drank a bottle” are simplified versions of the two verbal phrases of the compound predicate (the part of the sentence that describes what John is doing).
John ate a slice and drank a bottle.
This basic sentence structure should not include a comma. It is not a question of style or preference. It is not a controversial grammatical rule. It is a very common mistake.
While we’re on the subject, here’s a similar mistake: John ate a slice of pepperoni pizza, and some potato chips. There shouldn’t be a comma there, either. Per Purdue: Don’t put a comma between the two nouns, noun phrases, or noun clauses in a compound subject or compound object.
Yet people may be so worried about all the times when commas should be used that they start making up their own instances.
Now there is a related structure in which commas should be used. This is when a comma separates independent clauses that are joined by a coordinating conjunction: John ate a slice of pepperoni pizza, and he drank a bottle of beer. Grammatically, this sentence is different from the original example, with two independent clauses (“John ate a slice” and “he drank a bottle of beer”) separated by a coordinating conjunction (“and”).
Again, this sentence doesn’t take a comma: John ate a slice of pizza and drank a bottle of beer.
This sentence does: John ate a slice of pizza, and he drank a bottle of beer.
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