Should you have a comma before "because"?

In most circumstances, "because" is a subordinating conjunction, so when it starts a dependent clause after the main clause, it shouldn't be preceded by a comma. However, there are two exceptions to this rule:

• when the independent clause that comes before "because" contains a negative verb (e.g. don't, couldn't, wouldn't)
• when the independent clause that comes before "because" contains two elements and it is unclear which one "because" refers to.

Let’s look at an example of when the independent clause that comes before "because" contains a negative verb:

• He didn't take the exit because of the fog.

This example is confusing because you're unsure whether it was the fog that caused him to miss the exit, or some unknown factor. e.g. He didn't take the exit because of the fog, but because he wasn't paying attention.

Where you mean that the fog didn't cause him to miss the exit it might be best to rewrite it as: The fog didn't cause him to miss the exit. If you mean that he missed the exit because of the fog, then you should include a comma.

• He didn't take the exit, because of the fog.

Likewise, I couldn't come because of the traffic is ambiguous. These sentences are better:

• I couldn't come, because of the traffic.
• I couldn't come because of the traffic, but because my car broke down.
• It wasn't the traffic that meant I couldn't come, it was because my car broke down.

Remember, any sentence that causes ambiguity in your readers' minds stops them understanding your message. If you have an ambiguous sentence, see if it needs a comma, a rewrite, or both!