>>713192241"u" when pronounced as "yu" is literally the only way this happens, for example "unilateral" is pronounced "yu-nee-la-ter-al" so it's "a unilateral" where as words with "u" like "umbrella" or "underwater" are pronounced with "uh"
anytime it sounds like "uh" you use "an" so "an umbrella" or "an underwater cave" and when it sounds like "yu" it's "a unilateral agreement" or "a universe filled with stars"
every other vowel be it pronounced in it's lower or upper casing follows "an" so "an elephant" which starts with pronouncing "eh" or "an eel" which starts with pronouncing a hard "ee"
it's not that hard really, english is a language you're better off learning by listening and speaking the language instead of reading at least that's how everyone i know has done thanks to american and british tv shows
here is when i find out i'm wrong and look like a fool