The proper adjective form is European. (It is also the demonym for a person from Europe.)
Yes, European is a proper adjective that describes a noun as of or from Europe.
The adjective form of America is American. The word American can also be a noun (demonym).
No, European is an adjective, the corresponding proper noun is Europe.
Middle Eastern is the only proper adjective in the sentence above. It describes "greenhouses."
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
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.