the article 'a' is used for words starting with a consonant sound ( not always a consonant letter) while the article 'an' is used for words starting with a vowel sound (not always a vowel letter). the English language has 5 vowels and 21 consonants from which 20 vowel sounds and 24 consonant sounds can be produced.
the letters 'u' and 'h' usually confuse many learners. 'u' has a consonant sound in universal, union and urinate but has a vowel sound in uncle, umpire, until.
As for the letter 'h', it does not have a vowel form but is silent in words like honour, honesty and hour. this way, the vowel sounds start the words making it possess the 'an' article.
U is pronounced /ju/. the consonant sound there attracts the 'a' article.