Caribbean
The proper noun Caribbean refers to the region of the Caribbean Sea. The word is widely used as an adjective to refer to people of things in or from the region. The noun Caribbean can also refer to a native Carib islander.
When a proper noun is used as and adjective, it is a proper adjective; for example:Ancient Mexican structures have similarities to ancient Egyptian structures.
They are all nouns, Caribbean being a form of "Caribbean Sea" or "Caribbean region." But Caribbean can also function as a proper adjective (Caribbean tribes, Caribbean music).*The word island is usually considered a noun adjunct (attributive noun) when used with another noun, e.g. island paradise, island map. This also applies to location in location scouting and reunion in reunion committee.
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
"Trinidadian" is a proper adjective, as it derives from a proper noun, specifically referring to Trinidad, which is a country in the Caribbean. Proper adjectives are often used to describe something related to a specific place, culture, or nationality. In this case, "Trinidadian" describes people, culture, or things originating from Trinidad.
Vietnamese is the proper adjective for Vietnam.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.
The proper adjective for George is Georgian.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.