Concrete:
* When referring to a person: "The American gave me directions." Here, "American" refers to a specific individual and is therefore concrete.
* When referring to something tangible associated with America: "I bought an American flag." Here, "American" refers to a physical object and is concrete.
Abstract:
* When referring to the concept of being American: "He was proud of his American identity." Here, "American" represents an abstract concept of nationality and is therefore abstract.
* When referring to a quality or characteristic of America: "The American way of life is diverse." Here, "American" represents an abstract concept of culture and is therefore abstract.
It's important to understand the specific context in which the word is used to determine whether it is functioning as a concrete or abstract noun.