"Ralph" is the subject of the sentence. " The verb "put" is transitive here and being transitive, it asks for a direct object, which is a noun. " Logic aside (one would more easily put a note than a door somewhere), we are immediately keyed into "the note" as the direct object of the verb, and "on the door" as something other than the direct object, since there is a preposition present, and there would be no preposition attached to a direct object.
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