You don’t automatically put a comma before the word "because," but sometimes you need a comma there to make sure your meaning is clear. Here are some examples.
A reader named JC asked when he needs a comma before the word "because." He wondered about this exchange:
What did you like about camp?
I liked swimming and hiking because they were fun.
Does he need a comma before "because they were fun"?
The short answer is no.
It’s unusual to put a comma before "because." You only do it when you need the comma to prevent confusion because your sentence could have two meanings.
The Chicago Manual of Style has an excellent entry on this topic in its Q&A section online. It gives the example
He didn’t run because he was afraid.
Without a comma, you don’t know whether the writer means that the reason the man didn’t run was that he was afraid or whether the writer means there was some different reason the man ran.
If you put a comma before "because" ("He didn’t run, because he was afraid"), it’s clear that the part after the comma is extra information: the reason the man didn’t run—because he was afraid.
If you leave out the comma, you should probably add clarifying information to the end of the sentence. For example, you could write,
He didn’t run because he was afraid; he ran because the fire made his hiding place too hot.
You can imagine other sentences in which "because" may be ambiguous:
She didn’t want to cook because it was her birthday. (Does she want to have a break from cooking on her birthday, or does she want to cook for some other reason? She didn't want to cook because it was her birthday; she wanted to cook because she likes her own cooking better than restaurant food.)
Often a sentence that needs a comma before "because" will start with a negative statement, like both our previous examples—"He didn’t run" and "She didn’t want to cook"—but sometimes a positive sentence needs a comma too. Consider this example:
I heard Marylou got fired because Bob was gossiping in my dad’s store.
Did Marylou get fired because of Bob’s gossiping or did the writer hear about the firing from Bob’s gossiping? It’s not clear without a comma. If you put a comma before "because," it’s clear that the writer heard about it from Bob. If you mean that gossiping was the cause of the firing, it’s best to reword the sentence to something like "I heard Marylou got fired because she couldn’t stop Bob from gossiping in my dad’s store."
These are the unusual sentences that need a comma before "because." More often, you’ll have a simple sentence like JC’s: "I liked swimming and hiking because they were fun." It’s unambiguous without the comma, so you don’t need one.
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