Definite and indefinite articles
The definite article is “the”. “A” or “an” are the indefinite articles. Substantives may also appear without any article being applied to them, as is the case with this sentence, in which “substantives” is not modified by either kind of article.
This is one of the most frustrating topics for those whose mother tongue is something other than English. I know that I follow very definite rules for the proper use of articles, but I learned the rules by trial-and-error rather than by consulting a grammar.
Here are three pages that turned up in a search. They provide a good starting-point but do not provide an entirely identical set of rules:
Butte College, "Definite and Indefinite Articles"
Wall Street English, "The Complete Guide to Definite and Indefinite Articles"