grammar.articles.el-with-feminine-nouns
El agua, el águila: el before feminine nouns
Feminine nouns starting with stressed a-/ha- take el/un in the singular for sound, but stay feminine: el agua fría, las aguas frías.
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Explication
A feminine noun that begins with a stressed a- or ha- takes el (and un) in the singular, purely for sound — la agua would be awkward to pronounce. So it's el agua, el alma, el hambre, el águila, el área, un arma. The noun stays grammatically feminine throughout.
Because the gender doesn't change, every agreement is feminine: el agua está fría, esta agua, mucha hambre, el águila majestuosa. Only the article directly in front of the noun swaps to el.
Two limits: it applies only in the singular (the plural is regular: las aguas, las águilas) and only when the article is adjacent to the noun — la misma agua, la mejor área, because an adjective intervenes. Words whose initial a is unstressed keep la: la arena, la amiga, la harina.
Exemples
The water here is drinkable.
Région: global
I'm very hungry.
Région: global
The area is large, but the neighbouring areas are small.