adjectival vs. adverbial prepositional phrases

I have noticed that in a lot of the languages that I have looked at,
little grammatical distinction is made between prepositional phrases
modifying nouns and those modifying sentences as a whole (or their
predicates).

For example, in the sentence "The presents under the tree are for your
sister," the prepositional phrase "under the tree" is adjectival.  In "I
sat under the tree and opened the presents," it is "adverbial" (or
expresses the grammatical role Location).  Word order and distinctions
between parts of speech clarify the meaning.  Roughly, if a prepositional
phrase follows a noun it is probably adjectival, and if it does not follow
a noun it is probably adverbial/grammatical-role.  Of course, there are
also plenty of ambiguous examples like "I opened the presents under the
tree."

Upon consideration, it seems to me that, while there is a class of
prepositions that can occur in both guises, many other prepositions are
pretty strictly either adjectival or adverbial.  For instance, the
possessive "of" is strictly adjectival, while the instrumental "with" ("I
hit it with a hammer") is strictly a grammatical role.  My examples are
significant; the adjectival/role distinction can be made with traditional
cases too.

Aren't we dealing with two different but related grammatical concepts
here?

In my conlang, Bendeh, I have two sytactic structures for location.  The
prefix-suffix pair "oge--yk" marks a location modifying a noun, and is
formed the other adjectival prepositions, while the prefix "gy-" marks the
location of an entire clause, and looks like the other grammatical roles.

mug = book
cus = house
Lajezxnasx emug ogecUsyk. = I read the book (that was) in the house.
Lajezxnasx emug gycus. = I read the book (while I was) in the house.

The choice of the same consonant ("g") is significant.  There are other
pairs with similar meanings, like the adjectival "for" ("This book is for
Mary") and the Purpose role ("I read the book in order to please Mary").
I made these sound alike too.

Has anyone else addressed this problem in their projects?  What solutions
you have come up with.

- Nathaniel Lew

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