The indefinite articles a and an are sometimes misused, most often when combined with words starting with an h or o. The grammar rules below explain how to use these indefinite articles correctly.
1. Grammar Rules: A or An?– Choose the indefinite article a before words that begin with consonant sounds, including words that start with vowels.
Consonants are letters other than vowels: b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, (sometimes y), and z.
The following words begin with consonant sounds and start with consonants: ball, car, door, fire, garage, horse, jar, kite, lantern, mop, nurse, pizza, queen, rose, sign, tiger, vase, war, xylophone, yacht, and zebra. Comparatively, one-time and one-sided are words that begin with a consonant sound (w) but start with a vowel (o).
The indefinite article a precedes all of the above words because each begins with a consonant sound: a ball; a car; a door; a fire; a garage; a horse; a jar; a kite; a lantern; a mop; a nurse; a one-time charge; a one-sided argument; a pizza; a queen; a rose; a sign; a tiger; a vase; a war; a xylophone; a yacht; and a zebra.
2. Grammar Rules: A or An?– Choose the indefinite article an before words that begin with vowel sounds, including words that start with consonants.
The six vowels are a, e, i, o, u (and sometimes y). The following words begin with vowel sounds and start with vowels: ape, apple, astronaut, eel, elephant, equestrian, idea, illness, image, ode, offer, opera, ulcer, uncle, and umpire. Contrastingly, honest and honor are words that begin with a vowel sound (o) but start with a consonant (h).
The indefinite article an precedes all of the above words because each begins with a vowel sound: an ape; an apple; an astronaut; an eel; an elephant; an equestrian; an honest person; an honor; an idea; an illness; an image; an ode; an offer; an opera; an ulcer; an uncle, and an umpire.
“Articles: A versus An,” Purdue Online Writing Lab, Purdue University.
“Indefinite Articles,” Oxford Dictionaries.