It's not quite as simple as 'a' before consonants and 'an' before vowels. It's about whether the word starts with a vowel sound or not (which isn't quite the same thing as starting with a vowel.) For example, 'university.' 'U' is a vowel but this word starts with a 'y' sound so it always goes with 'a.' 'Hour' is a good example the other way around. 'H' is a consonant but here it is silent and consequently we say 'an hour' and not 'a hour.'

"a" is used before consonants such as "b,c,d,f,g,h" "an" is used before vowels such as "a,e,i,o,u" (and also "y" if it is being used as a vowel)

'An' is used before words which begin with vowel sounds, usually a, e, i, o, u. an apple an envelope an ice cream an orange an umbrella 'An' is also used before the letter 'h' when it is silent: an hour (this sounds like 'an-our', because the 'h' is silent). an honest person 'An' is also used before certain letters and numbers: An FBI agent (because the name of the letter 'F' is sounds like 'ef') An MBA student (because the name of the letter 'M' is sounds like 'em') An 18th-century novel (an eighteenth..) 'A' is used before consonant sounds, b, c, d, f, g, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z, and 'h' when it is not silent: a book a jungle a hamburger 'A' is also used also before words beginning with vowels when they are pronounced as consonants: a university a European city I hope that helps.