|Have you ever seen a masculine article ("un", "el") before a feminine word? Maybe, you have seen something like "un alma" or "el aura". But all this does not make sense! "Alma" ("soul") is a feminine noun and same thing happens with "aura" (same in English: "aura"). And both are preceded by a masculine article. So according to the general rules, we should use a feminine article instead, as nouns and their corresponding articles must agree in gender and number.|
So, according to this, the right use should be "una alma" and "la aura". Right? But, no, it isn't so.
Let see what the Real Academia de la Lengua Española says about this:
"La forma femenina "una" se apocopa normalmente en "un" ante sustantivos femeninos que comienzan por /a/ tónica: un águila, un hacha (aunque no se considera incorrecto, hoy es infrecuente en estos casos el uso de la forma plena una: una águila, una hacha); pero si entre el indefinido y el sustantivo se interpone otra palabra, ya no se produce la apócope: una majestuosa águila, una afilada hacha. También cuando el adjetivo va pospuesto debe concordar en femenino con el sustantivo: un águila majestuosa, un hacha afilada (y no un águila majestuoso, un hacha afilado)."
"The female form una normally becomes "un" before feminine nouns beginning with tonic / a /: un águila, un hacha (although not considered wrong, it is nowadays rare in these cases the use of "una": una águila, una hacha), but if another word is placed between the indefinite and the noun , it doesn't no longer produces the short version: una magestuosa águila (a majestic eagle), una afilada hacha (a sharp ax). Also, when the adjective is postponed, it must agree in feminine gender with the noun: un águila magestuosa (a majestic eagle), un hacha afilada (a sharp ax) (not un águila majestuoso, un hacha afilado."
Ok, ok, don't panic!
Let's understand this with some examples:
First thing: "una sílaba tónica" (a tonic syllable) is the syllable taking the strength when the word is pronounced. So, whenever we find a word which first syllable starts with vowel "a" and being that syllable tonic, we should use a masculine article (el, un). The RAE says this is not obligatory, but it is better to do it, in order to avoid a rare pronunciation. Examples: "un asa" (a handle), where "asa" is a feminine noun, but, as the first syllable is tonic, and starts with an "a", we must use the masculine article ("un", in this case)
Second: Let's say we have "un asa". If we place an adjective between "un" and "asa", there is no need to use the masculine article. So, "un asa" (a handle), but "una gran asa" (a big handle).
And finally, if the adjective is placed after the noun, instead of being in front of it, as in our last example, this adjective must agree in gender (and number) with the noun. Also note than in this case, as there is no adjective between the article and the noun, the article will be masculine. Examples:
"el ala blanca". Here, "blanca", the adjective, is placed after the noun "ala". "Ala" is a feminine word, so the adjective placed after it must also be feminine.
So. in few words:
la blanca ala
el ala blanca
For more information about gender of Spanish nouns, the book Practice Makes Perfect Complete Spanish Grammar, 2nd Edition (Practice Makes Perfect Series)
(concretely units 17 an 18) is a great resource.
Hope this helps!