People taught me the a/an rule based on the vowel, but never taught me about this, probably because at school, the emphasis is on grading on the written languag

People taught me the a/an rule based on the vowel, but never taught me about this, probably because at school, the emphasis is on grading on the written language. I just noticed that I do this change in pronunciation without even (and ever) noticing it, which is how I like to approach learning these rules, by pure exposure.