If you can answer the question,
"Which...?" about the word in italics,
choose the.
If you cannot answer the question,
"Which...?" about the word in italics,
choose a / an or Ø (no article).
Note
Adjectives do not usually define a noun; they create a sub class.
"German beer" is a sub class of "beer".
Only superlative adjectives define a noun - because there is only one of a class that is the best, biggest, longest, etc.