9. I'm thinking of buying a present for my son's birthday.

(Does 'for my birthday' modify 'bying' and is it an adverbial clause as well? Or is it an adjective phrase modifying 'present'?))

10. I need two dollars for the bus.
(Does 'for the bus' modify 'need' and is it an adverbial clause as well? Or is it an adverbial phrase modifying 'two dollars'.)

1 answer

9. "for my son's birthday" is serving as an adjective phrase modifying "present," yes. It's a phrase because there's no subject/verb in it, and it's an adjective phrase because it's modifying/describing a noun.

10. In this sentence the prepositional phrase seems more adverbial because it's telling why it's needed; it's not describing the bus. (PS. Adverbs don't modify nouns, which is what "dollars" is.)

Adverbs answer one of these four questions: How? When? Where? and Why?
http://www.chompchomp.com/terms/adverb.htm