The definite/indefinite article -- the/a(n) -- always comes before a noun and can never be used without a noun.
Is the definite/indefinite article a complement or a modifier of a noun?
A modifier changes the meaning of an element. https://en.wikipedia.org/wiki/Grammatical_modifier
An article is a determiner. It serves to express a noun's or a noun phrase's reference in the context. https://en.wikipedia.org/wiki/English_determiners