That only happens with a handful of words: those that 1) are feminine and 2) their first syllable begins with an "a" and 3) their first syllable is stressed.
It's similar to English which uses "an" instead of "a", but it happens very very very rarely.
(Needless to say, "águila" and "ala" ARE feminine nouns, what's changing here is the determiner so the two a's don't clash, not the gender of the nouns)
It's similar to English which uses "an" instead of "a", but it happens very very very rarely.
(Needless to say, "águila" and "ala" ARE feminine nouns, what's changing here is the determiner so the two a's don't clash, not the gender of the nouns)