A" and "an" are indefinite articles that precede nouns or the adjectives modifying nouns. In English grammar, "a" and "an" are determiners, which means they specify the identity or quantity of some thing, and for the two phrases, that quantity is "1"—the word from which they're derived. You simply use https://lukaslqvad.wikiparticularization.com/678121/strathmore_fundamentals_explained