What are articles? Basically, an article is an adjective. Like adjectives, articles modify nouns. We divide them into "indefinite" and "definite"
We use "indefinite" to mean not sure, not certain. "Indefinite" is general.
We use "definite" to mean sure, certain. "Definite" is specific.
For my birthday I want the brown teddy bear.
I have specified what present I want for my birthday. (definite)
For my birthday I want a toy.
It can be a ball, a teddy bear, a bike, etc (indefinite)
Use "a/an" with singular nouns. Ex: a pencil. Not: a pencils.
Use "the" with singular and plural nouns. Ex: the pencil, the pencils.
Use "an" with a noun beginning with vowel. (a,e,i,o,u) Ex: an arm.