Using "A" and "AN"

Using "a" and "an"
The choice between "a" and "an" depends solely on the sound that immediately follows it.
"a" is used before words beginning with a consonant sound

(the "y" and "w" sounds, as in "yellow" and "worm," count as consonants for this purpose)

a cat

a dog

a worm

a yellow balloon

a Euclidian theorem

a European country

a unique problem

a Ouagadougou resident (Ouagadugu is pronounced as though it were spelled "Wogadugu")

"an" is used before words beginning with a vowel sound
an apple

an orange

an hour

an honest man

an honorable man

an utter waste

an yttrium compound (yttrium is pronounced as though it were spelled "ittrium")
The only modern cases of disagreement:
Some words are pronounced differently in British and American English. An American would say "an herb garden" (because the "h" is silent in American pronunciation); a British person would say "a herb garden" (because the "h" is not silent in British pronunciation).
In British usage, "an" is used before a word beginning with "h" if the principal stress in the word does not fall on the first syllable, even if the "h" is not normally silent. E.g., "a happy birthday," "a history book," but "an historical event."
In American usage, "h" at the beginning of a word is either silent or not, regardless of the stress pattern. E.g., "a happy birthday," "a history book," "a historical event."