Q: I’m in the editing phase of a book and notice that the copyeditor has added a comma before many (perhaps most) occurrences of the word “because.” This seems to halt the flow of the sentences, but I wonder if a rule exists that I’m not aware of.
A: Generally, according to the few usage guides that comment on this issue, a comma is not used before “because.”
The comma is appropriate only when needed to guide the reader through an unusually long or complex sentence.
The conjunction “because” generally introduces a dependent clause, one that adds information to the main clause. The “because” clause provides a reason, cause, or motive—the “why” of the sentence.
Normally, the “because” part flows smoothly from the main clause and no break is needed between them: “He ran because he was a coward” … “They moved because they’re expecting a third child” … “She’s upset because you left.”
However, according to some usage guides, you do need a preceding comma if the clause starts with “perhaps because” or “possibly because” or “but not because” (“He ran, perhaps because he was a coward”).
And you need a following comma, according to these usage authorities, if the “because clause” comes first (“Because he was a coward, he ran”).
You’ll find advice along these lines in Garner’s Modern American Usage (3rd ed.) and The New York Public Library Writer’s Guide to Style and Usage.
Note that a sentence with a negative verb followed by “because” can easily be misread: “He wasn’t fired because he was late.” Was he fired or wasn’t he?
Do the reader a favor and clarify the situation: “He wasn’t fired because he was late. He was fired because he embezzled money.”
That kind of negative sentence, by the way, can be changed completely if a comma is thoughtlessly added.
For example, these two sentences have opposite meanings: “She didn’t marry him because he was dying” … “She didn’t marry him, because he was dying.”
In the first sentence, she married him—but not because he was dying. In the second sentence, she didn’t marry him—because he was dying.
If the first sentence is what you mean to say, it’s better to rewrite it and avoid the ambiguity: “She married him, but not because he was dying.”
It’s always worth taking one more look at those commas!
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