This is probably too difficult for elementary unless you will gloss over it, they most likely won't get the concept.
I had to teach it to 2nd year High School, and many had issues...even with me explaining in Korean sometimes.
Definite Article - 정관사 The
Indefinite Article - 부정관사 A / An
The reasons for the difference in use varies....but basically it depends
1. Countable or Non countable nouns
The air, the oil, the sand , the weather
A ball, a pen, a car etc
2. If the noun is 'specific' it becomes the,
I watched a film - what film ? any
I watched the film you recommended ? a specific film
A slice of pizza, the smallest slice
3. In a story, when you are first introduced to characters or places, it's A/AN, afterwards it's The.
A man lived in a town. (a man, we don't know him, 1st time)
The man had a dog (The man, already introduced, 2nd time)
The dog was black
You can see how and why this is difficult to teach properly.
You then also have the difference between A / AN ....which at it's most basic for is to do with vowels and their sounds. But there are exceptions such as an X-Ray (because X in this case takes a E sound)
Personally I'd forget teaching Articles to elementary students
