From the Tips and Notes: Adjectives and Definite Nouns
While nouns normally express definiteness using a postfix, this changes to using an article if any adjectives is attached to the noun.
If the adjective is used with a definite noun, then it is put between the definite article and the noun: En åben bil (an open car) becomes den åbne bil (the open car). In this case the adjective is declined the same way as for the plural, no matter the grammatical number or gender of the noun.
From what I understand...
Indefinite noun: She is an interesting girl = Hun er en interessant pige.
Definite noun: She is the interesting girl = Hun er den interessante pige.