is the word "a", and the word "an" in English

indicates that the noun refers to a general thing (Sit on a chair) - means sit on any chair

is used as a synonym for the number one, as in "make a wish", "a hundred", "an apple" (an is an older spelling of "one")

is used to express a proportional relationship, such as "a dollar a day" or "$50 an ounce"

use "a" before a noun beginning with a consonant sound (a chair, a table)

use "an" before a noun beginning with a vowel sound (an apple, an orange)