Here's why:
* Common noun: Refers to a general category of things, rather than a specific individual or item. "Agenda" refers to any list of things to be done.
* Proper noun: Refers to a specific person, place, or thing (e.g., John, London, Eiffel Tower).
* Collective noun: Refers to a group of people or things (e.g., team, flock, committee).
Let me know if you have any other questions!