In Spanish, few nouns can stand alone. Most need to be introduced or determined by an article. As in English, an article is characterized as either definite (the) or indefinite (a, an). Spanish articles must match the gender and the number of the noun they determine, resulting in four forms of the indefinite article, as seen in the table above.
Masculine Article with A-initial Feminine Nouns
When a feminine singular noun begins with a stressed –a or –ha, the masculine article un or el is used instead the feminine una or la, to aide in pronunciation. When the same noun is plural, the feminine article unas or las is used. The following feminine nouns use the masculine article when singular.
un alma – unas almas soul
un hacha – unas hachas ax
un hambre – unas hambres hunger
un agua – unas aguas water
un águila – unas águilas eagle
As the English a, an, or some, the indefinite articles un, una, unos, unas refer to nouns which are non-specific. Un or una may also indicate quantity, meaning one. Compare the use of the indefinite and definite articles in the first two sentences below.
Tenía un pingüino yo como de mascota…. me iba al supermercado y compraba pescado y le daba el pescado, porque no comía nada, uno de los compañeros de trabajo de ahí del local, me dijo el pingüino no va a comer nunca un pescado muerto.
I had a penguin as a pet… I would go to the supermarket and buy him fish and I would give him the fish. Because he wasn’t eating anything, one of the staff of the store told me the penguin is never ever going to eat a dead fish.
With Adjectives of Profession, Nationality, and Religion
Professions, nationalities, and religions are considered adjectives in Spanish and need no article after verbs like ser.
A mí me encantaría ser arquitecto.
I would love to be an architect.
Tengo una compañera de trabajo, ella es maestra.
I have a collegue; she is a teacher.