FANBOYS – COORDINATING CONJUNCTIONS (for and nor but or yet so) – always second, always with comma. I work, and my wife goes to school. I work, but I don’t get much money.
SUBORDINATING CONJUNCTIONS (because, if, unless, when, although, before, while…) – can be first or second. Use a comma only if the dependent clause is first.
I will be there unless it rains. She never smokes because it will make her sick.
Unless it rains, I will be there. Because it will make her sick, she never smokes.