Feminine nouns that start with a stressed “a” in singular take another variant of the feminine article, its form is just like the masculine form: el; both forms have origin in the Latin word illa.
The apócope (deletion of a final sound) with indefinite article happens just as if that word was masculine: un (and ningún/algún), unless split by some adjective. Adjectives are always feminine as they have to match the gender:
el agua ✅
la agua ❌
las aguas ✅
el agua frío ❌
el agua fría ✅
el ave roja ✅
la roja ave ✅
el águila calva ✅
ninguna agua ❌
ningún arma ✅
algún arma ✅
este arma ❌
esta agua ✅
otra ave ✅
¡Ojo!
Adding a suffix may shift the stress:
el agüita ❌
la agüita ✅
* The word azúcar is either feminine or masculine. It can take either masculine or feminine articles, even though the accentuated syllable is zú:
-
el azúcar / los azúcares
-
la azúcar / las azúcares
-
- el azúcar / las azúcares
Newly created words do not necessarily follow this rule, e.g. la árbitra.
sources: https://rae.es/dpd/el#21