A vs. An

If you’re a non-native English speaker you might have struggled about when to use a vs. an. A previous adviser of mine, after years of corrections, finally managed to make me internalize the basic rule:

If the following word starts with a vowel, use an. If the following word starts with a consonant, use a.

That’s not the actual rule though. The actual rule is

If the following word starts with a vowel sound, use an. If the following word starts with a consonant sound, use a.

You’re confused? Good. Thank Purdue for the easily accessible answer.