0. The rule
If a feminine word begins with a stressed /a/
sound, some singular articles/determiners:
take a masculine form:
if they occur directly before the noun.
1. i) Affected nouns
a. Words with stressed initial /a/
:
- Asia, acta, agua, ala, alga, alma, alza, ama, ancla, anda, ansa, ansia, ara, arca, arda, aria, arma, arpa, arria, asa, ascua, asna, aspa, asta, aula, aura, aya, ave, arte (m/f)
- África, ácana, áfaca, ágata, ágora, águila, álaga, álala, álgebra, álica, álula, ámpula, áncora, ánfora, ánima, árdea, área, árgana, árgoma, árnica, árula, áspera, ática
- haba, habla, haca, hacha, hada, halda, hampa, harca, harda, harma, harpa, harria, haya, haza, hambre, haz
- hálara
b. Exceptions:
- Letters: la a, la hache, la alfa, el/la álef
- Acronyms: la ASA, la APA etc
- Proper names: la Ana, la Ángela, la Austria, la Ávila, la Ámsterdam, la Haya etc
- Exception to exception: Continents: el Asia, el África
- Nominalized adjectives: la alta, la árida etc
- (Some) fem. nouns with fem. referents: la árbitra
- Common gender nouns with fem. referents: la árabe, la ácrata, la alto (singer)
1. ii) Affected articles/determiners
The rule for which articles/determiners take 'masculine' forms with this class of words appears to be:
- if the apocopated form of the feminine word is identical to the masculine word, then the masculine form is used:
e.g. una águila → un' águila = un águila
- if they are not identical, the feminine form is used:
e.g. esta águila → est' águila ≠ este águila
- exception: la → l' → el
Examples
el águila | las águilas | la gran águila
un águila | unas águilas | una gran águila
algún águila | algunas águilas | alguna gran águila
ningún águila | ningunas águilas | ninguna gran águila
-----------------|---------------------|--------------------
esa águila | esas águilas | esa gran águila
esta ... | estas ... | esta gran ...
aquesta ... | aquestas ... | ...
aquella ... | aquellas ... | ...
nuestra ... | nuestras ... | ...
vuestra ... | vuestras ... | ...
otra ... | otras ... | ...
poca ... | pocas ... | ...
toda ... | todas ... | ...
misma ... | mismas ... | ...
mucha ... | muchas ... | ...
tanta ... | tantas ... | ...
cierta ... | ciertas ... | ...
demasiada ... | demasiadas ... | ...
cada ... | | ...
| varias ... |
| sendas ... |
| ambas ... |
2. Descriptive: how native speakers treat agua-like nouns
As the comments on cdlvcdlv's answer show, among native speakers there is a tendency (despite the RAE's prescriptions) to treat words like agua as 'hermaphroditic' - postnomial modifiers are consistently treated as feminine, but prenomial modifiers show considerable fluctuation in gender.
This treatment of agua-like words isn't limited to casual speech, but is also found in educated speakers and formal/academic writing:
¿Con qué derecho se la toma por aquelm asa y no por éstaf?
... los desperfectos que elm abundante agua caídaf provocó en las vías...
... nuestrom habla riojanaf actual...
- Martínez Sáenz de Jubera y González Perujo, 1998
The first thing to note is that variation in gender agreement with nouns of this class generally affects determiners and adjectives preceding, but usually not following, the noun. For example, a search of 20th century texts in Corpus del
español (Davies n.d.) returns examples such as mismo agua 'same water', propio agua 'own water', mucho agua 'a lot of water', but no examples where a postposed adjective with agua is masculine. Counting both 19th and 20th century examples there are three tokens of buen alma ‘good soul’ in the corpus versus two of buena alma, whereas no tokens are found of alma bueno in comparison to 17 for alma buena. That is, for some speakers some of these nouns variably condition one type of gender agreement with prenominal modifiers and the other type with postnominal modifiers.
The hypothesized reasoning for native speakers internalizing this hermaphroditic treatment of agua-like words is the learning of tokens such as el agua and agua fría (recognising el as a masculine article and not an allomorph of la) and analogy to other such words.