The word a (which becomes an when the next word begins with a vowel a, e, i, o, u) is called the indefinite article because the noun it goes with is indefinite or general. The word the is known as the definite article and indicates a specific thing.
Use before unsounded ‘h’. Because the ‘h’ hasn’t any phonetic representation and has no audible sound, the sound that follows the article is a vowel; consequently, ‘an’ is used.
An honorable peace, an honest error.
When ‘u’ makes the same sound as the ‘y’ in ‘you’, or ‘o’; makes the same sound as ‘w’; in ‘won’; then ‘a’ is used. The word-initial ‘y’; sound (‘unicorn’) is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring ‘a’.
A union, a united front, a unicorn, a used napkin, a US ship, a one-legged man.
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