Here's why:
* Subject: The subject is the noun or pronoun that performs the action of the verb.
* Verb: The verb is "saw".
* You: This is the pronoun performing the action of seeing.
While "flock" and "geese" are nouns, they are objects in this sentence. "Flock" is the direct object, and "geese" is part of the prepositional phrase "of geese" which modifies "flock".