comma before because
When the adverbial clause after because takes a comma.
Because is a subordinating conjunction. It introduces an adverbial clause of reason. If that clause is restrictive, use no comma: They left because the hall was cold. If the clause is nonrestrictive, set it off with a comma: Dana signed the card, because someone had to.
After a negative, because is a special case. Without a comma, The board didn’t hire her because she knew the chair can mean they hired her for some other reason. With a comma, they did not hire her, and the tie is the reason: The board didn’t hire her, because she knew the chair.
Examples
UseThey left because the hall was cold.
UseDana signed the card, because someone had to.
UseThe board didn’t hire her, because she knew the chair.
AvoidThe board didn’t hire her because she knew the chair. (If you mean they refused her.)