Listener and Speaker
Every conversation consists of two parties: a listener and a speaker (you are always the speaker).
When choosing between A and THE you should consider whether the listener knows about the thing you are about to introduce:
- if the listener hears for the first time, doesn't know, you should use A: I saw a movie.
- if the listener hears for the second time, knows, you should use THE: I saw the movie.
In the case of “I saw a movie” you introduce the thing (movie) for the first time, the listener hears that information for the first time.
You know about the movie, you know because you saw it, but the listener doesn't, therefore, you should use A: I saw a movie = I saw one movie (any movie).
In the case of “I saw the movie” you introduce the thing (movie) for the second time, the listener knows about the movie you saw. Maybe the previous day you told the listener that you had a plan to see that particular movie, therefore, now the listener knows about which movie you are talking. I saw the movie = I saw the movie I told you about yesterday (one particular movie).
A = 1
THE = the 1 I told you about
Notice the interplay between A and THE when telling a story:
I met a girl yesterday. The girl was beautiful. She was wearing a long dress and a pearl necklace. The dress was black and the necklace was white. I decided to take her to an Italian restaurant near the Empire State Building. Unfortunately, the restaurant was closed so we went to the movies instead. We saw an interesting movie. The movie was about a young couple on their first date. The couple decided to go to an Italian restaurant near the Empire State Building. Unfortunately, the restaurant was closed so they went to the movies instead…
You introduce the person (girl, couple) or thing (dress, necklace, restaurant, movie) with A (listener hears for the first time) and follow it up with THE (listener hears for the second time).