>Michael,
    I like Bruce's explanation.  Functional grammar (SFG) would call it an
"existential" clause.  "There" is only a placemarker.  "A very big
difference between coca and cocaine" is the subject, and about all
we're predicating is that the difference exists.
   "Is" isn't really a linking verb here because nothing is being linked.

Craig
> How would you diagram "There is a very big difference between coca and
> cocaine"?
>
> Specifically, would you see the prepositional phrase as modifying
> "difference" or "is"?
>
> Thanks.
>
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