Please correct me if I say something dumb:
 
My understanding is that "because" is a conjunction setting forth a
cause-and-effect relationship, as in:
 
"I flunked because I didn't study."  -- or --
"Because I didn't study, I flunked."
 
... and "since" is an adverb setting out a time frame:
 
"I haven't studied since January."
"Since I got married I haven't had any fun."
 
I've always been corrected when I use constructions like:
 
"Since you're finished, we should leave now."  I had at least one
instructor who took great joy in x-ing out "since" and replacing it with
"because."
 
HOWEVER (isn't there always a however?), in drawing up a lesson plan,
I came across a citation in Webster's New World Dictionary, Warner
Books Paperback Edition of 1990, that lists "because" as a definition of
"since."  And now that I think about it, I see it used everywhere --
textbooks, scholarly papers, literary criticism.
 
What can I tell my students?  Is there any consensus out there?
 
Thanks.
_______________________________________________
Brenda S. Campbell
Wolf, Greenfield & Sacks, P.C.
600 Atlantic Ave.
Boston, MA 02210
(617) 720-3500 (voice)  (617) 720-2441 (fax)
_______________________________________________