Hi there,
There is a question in Chicago Manual Style book. But I didn't understand the explanation. Anyone helps?
Q. When do you use a comma before “because”? I feel that I never need to put a comma before “because” because any information after it is necessary. What are your thoughts?
A. I disagree. Here's the old example that comes to mind:
He didn’t run, because he was afraid.
He didn’t run because he was afraid.
In the first sentence, “because he was afraid” isn’t necessary; the main thing is that he didn’t run, and the reason is incidental. The second sentence, which omits the comma, is unclear. It might mean that he ran, but not because he was afraid. To prevent confusion, sometimes you need the comma.