Feminine nouns beginning with stressed /ˈa/ like es-note-noun-f-starting-with-stressed-a take the singular definite article el (otherwise reserved for masculine nouns) instead of the usual la: el es-note-noun-f-starting-with-stressed-a. This includes the contracted forms al and del (instead of a la and de la, respectively): al es-note-noun-f-starting-with-stressed-a, del es-note-noun-f-starting-with-stressed-a.
These nouns also usually take the indefinite article un that is otherwise used with masculine nouns (although the standard feminine form una is also permitted): un es-note-noun-f-starting-with-stressed-a or una es-note-noun-f-starting-with-stressed-a. The same is true with determiners algún/alguna and ningún/ninguna, as well as for numerals ending with 1 (e.g., veintiún/veintiuna).
However, if another word intervenes between the article and the noun, the usual feminine singular articles and determiners (la, una etc.) must be used: la mejor es-note-noun-f-starting-with-stressed-a, una buena es-note-noun-f-starting-with-stressed-a.
If an adjective follows the noun, it must agree with the noun's gender regardless of the article used: el es-note-noun-f-starting-with-stressed-a única, un(a) es-note-noun-f-starting-with-stressed-a buena.
In the plural, the usual feminine singular articles and determiners (las, unas etc.) are always used.
