NOUNS AND PRONOUNS - Obejects of prepositions

        S    V          O   prep    O of prep
(a) Ann put her books on the desk
                                            (Noun)
      S       V prep          O of prep
(b) a leaf fell to the ground.
                             (Noun)
Many English sentences have prepositional phrases. In (a): on the desk is a prepositional phrase. A prepositional phrase consists of a Preposition (prep) and an Object of a Preposition (O of Object).  is a noun. The Object of a Preposition is a NOUN.
Reference List of Prepositions:
About, above, across, after, against, along, among, around, at
Before, behind. Below, beneath, beside, between, beyond, by
Despite, down, during,
For, from
In, into
Like, near,
Of, off, on, out, over
Since,
Through, throughout, till, to, toward(s)
Under, until, up, upon
With, within, without.