Here's why:
* Objective Case: Used for objects of verbs and prepositions.
* Example: "I saw *them* at the park." (object of the verb "saw")
* Nominative Case: Used for subjects of verbs.
* Example: "*They* went to the park." (subject of the verb "went")
* Possessive Case: Shows ownership.
* Example: "Those are *their* bikes."
Let me know if you'd like more examples!