Here's why:
* Common nouns refer to general things or people, not specific ones. "Cupboard" refers to any type of cabinet used for storage.
* Proper nouns refer to specific people, places, or things (e.g., John, London, Eiffel Tower).
* Abstract nouns refer to ideas, concepts, or qualities (e.g., happiness, freedom, love).
Since "cupboard" doesn't refer to a specific item or idea, it's a common noun.