I reckoned you sure were Chinese immediately as I saw the question. I was taught some bs as well at school, "since, because, as, for, due to, by reason that etc". I think the teachers were just messing up with them, making no sense.
Sometimes those two are indeed interchangable.
Just follow your language "intuition", when you feel like an "由于,既然", use a "since" there, when you feel like an "因为", choose a "because". Yeah, that simple.