The article a becomes to an when the following noun starts with a vowel.
Not a adventure but an adventure.
But how is it, when there's an adjective before the noun?
Not a thrilling adventure or not an thrilling Adventure?
And how is it, when the adjective starts with a vowel but the noun does not?
Not a exciting tournament or not an exciting tournament?
A changes to an if the following word begins with a vowel sound. It doesn't matter what part of speech the word is.
Silent letters are ignored. So you say "an hour" because the "o" is a vowel sound after the initial silent "h." Similarly: "an honor, a house."
Note that the "long U" sound, as in *uniform" is not considered a vowel for this purpose, so you'd say "a uniform" but "an ugly duckling."
"Y" and "W", while they typically have vowel sounds, are not considered vowels for this purpose, so you'd say "a yellow flower," "a wish", etc. (There's rare exceptions with words that begin with "Y" that have a "short I" sound, but all such words would be names or scientific words.)
