Alex sent (transitive verb) a postcard (direct object) from Argentina. She left (transitive verb) the keys (direct object) on the table. My father took (transitive verb) me (direct object) to the movies for my birthday. Please buy (transitive verb) me a dog (direct object)!
Prepositions must always be followed by a noun or pronoun. That noun is called the object of the preposition. Note that a verb can't be the object of a preposition.