This will be an overgeneralization, but I'll state it anyway and assume
that if (when?) it's shot down, the wreckage might still be useful.
English grammars vary in how they use the term 'preposition', but any
given grammar will adopt one of two basic approaches (reflecting the
form/function distinction) and then edge away from it for problem cases.

(1) Use 'preposition' for a set of words that *can* be used to begin a
prepositional phrase -- regardless of the function a given instance is
serving in a particular sentence. 

(2) Use 'preposition' only for words that are beginning a prepositional
phrase.

Approach (1) would label the 'up' in "put up with" and the "up" in "I
looked up when I heard the noise" as prepositions. Approach (2) would
lead to different terms, e.g. "adverbial particle" in the latter
sentence. Of course, the second you try to provide a list of
prepositions, you're shoved toward (1), since you're almost completely
decontextualizing the words. 


Almost all grammars that adopt (1), though, make an exception for the
"beginning a subordinate clause" category. Thrax just had a "particle"
class, but once people started subdividing that, having prepositions and
coordinating conjunctions in separate categories made sense. But that
left subordinating conjunctions kind of floating in between....

Bill Spruiell

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