I was going to start by stating the difference is “a” is unspecific, “the” implies the subject is already known.
However as I thought more about it I realized it gets a little more complicated. I’m not academically studied in English, though I am a native speaker.
For example if you say, “Our town has a castle” - that’s a specific castle but you wouldn’t say “our town has the castle.”
Why? It has to do with what the listener knows. “Definite” means the subject is known to the listener. If I’m introducing a castle into the conversation that you don’t know, I’m talking about _a_ castle. Now that it’s introduced, I can tell you about _the_ castle.
However as I thought more about it I realized it gets a little more complicated. I’m not academically studied in English, though I am a native speaker.
For example if you say, “Our town has a castle” - that’s a specific castle but you wouldn’t say “our town has the castle.”
Why? It has to do with what the listener knows. “Definite” means the subject is known to the listener. If I’m introducing a castle into the conversation that you don’t know, I’m talking about _a_ castle. Now that it’s introduced, I can tell you about _the_ castle.