A and AN is used when we speak of one person, place, animal or thing.
AN is used before naming words that begin with the vowel sounds a, e, i, o
Example: an apple, an egg, an orange, an elephant
A is used before naming words that begin with the consonant sounds b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z
Example: a cat, a dog, a goat, a hat
Use of a and an in sentence:
This is an egg.
This is a goat
This is an igloo
This is an orange