posted by John .
* You need clean water for drinking.
In this sentence, what does 'for drinking' modify? Does it modify 'clean water' or 'need'?
Is 'for drinking' an adjective phrase or an adverbial phrase?
Let's see ...
... "water" is the direct object,
"clean" is an adjective modifying "water,"
and yes, "for drinking" modifies "water" also -- so it must be a prepositional phrase acting as an adjective.
I suppose it could be argued that it's adverbial, but I think it's adjectival because it's closest to "water." Usually, word order in an English sentence helps to determine meaning.