Here's why:
* Parts of speech are categories of words based on their grammatical function and meaning. Examples include nouns, verbs, adjectives, adverbs, pronouns, prepositions, conjunctions, and interjections.
* Nominative refers to the grammatical case of a noun or pronoun that functions as the subject of a verb.
* Object refers to a noun or pronoun that receives the action of a verb. There are direct objects (receive the action directly), indirect objects (receive the benefit of the action), and objects of prepositions (follow a preposition).
* Possessive refers to a grammatical case that indicates ownership or possession. It is typically expressed with an apostrophe and an "s" ('s) or just an apostrophe (').
In summary:
* "Nominative" and "possessive" are grammatical cases, not parts of speech.
* "Object" is a grammatical function, not a part of speech.
Example:
* The dog (nominative) chased the ball (object).
* "Dog" is the subject (nominative case) and "ball" is the direct object.
* The dog's (possessive) tail wagged.
* "Dog's" indicates possession.