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When to use a comma before because

Question:

I was told never to use a comma before a clause beginning with because. But someone told me I should have a comma in this sentence:

Ms. Harris was not defeated in the last election, because she changed her position on a crucial issue.

Can you explain why?

Answer:

A clause beginning with because gives the reason for the action in the sentence. As a result, it usually gives essential information and cannot be separated from the main part of the sentence by a comma.

However, a problem arises when the main verb is in the negative, as it is in your example: was not defeated. In that case, the meaning of the sentence depends on whether you use a comma.

With a comma

If you add a comma to the sentence, you create a separation between not and because:

Ms. Harris was not defeated, because she changed her position.

This sentence clearly means that Ms. Harris was not defeated in the election and that the reason for her success was the change in her position.

Without a comma

If you omit the comma, however, you create a close link between because and not:

Ms. Harris was not defeated because she changed her position.

The meaning here is unclear. Readers may think this sentence means that Ms. Harris was defeated in the election—but not because she changed her position. If that is your meaning, you should rephrase the sentence for greater clarity:

The reason for Ms. Harris's defeat was not the change in her position.