comma before because

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

• The cat went into the shed because it was frightened.
• Ms. Harris lost because she changed her position on a key issue.

However, a problem arises when the main verb is in the negative: e.g. did not go in, did not lose. 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:

• The cat did not go into the shed, because it was frightened.
• Ms. Harris did not lose, because she changed her position on a key issue.

These sentences clearly mean that the cat did not go into the shed and that Ms. Harris did not lose. The because clauses give the reason why the main action did not take place.

Without a comma

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

• The cat did not go into the shed because it was frightened.
• Ms. Harris did not lose because she changed her position on a key issue.

The absence of a comma reverses the meaning: in this case, the because clause is eliminating a reason for the action in the main clause. The cat did go into the shed—but not because it was frightened; and Ms. Harris did lose—but not because she changed her position.

Because the reader may not realize the significance of the absent comma, it may be better to reword these sentences to make the meaning completely clear:

• It was not out of fear that the cat went into the shed.
• Ms. Harris’ defeat was not due to the change in her position on a key issue.