Whether we are talking about sentences, paragraphs, or whole books, it
has always been clear to me that defining "the main idea" is hopeless,
since it has as much or more to do with the audience as it does with ther
speaker.  Consider a case in which young Jeremy, not a stellar student,
comes home and announces: "If I get A's on my nexct three French tests, I
can get a C in the course!"
 
Jeremy's father (whom Jeremy will remember as the kind of father for whom
Jeremy could never be good enough), answers, "That's a big 'if'!"
 
Jeremy's mother (a saint), answers, "A C in French -- won't that be
wonderful!"
 
For the father, the main idea was in the subordinate clause; for the
mother, it was in the main clause.  Neither structure nor placement has
anything to do with it!
 
Michael Kischner