An indefinite article, called an artículo indefinido in Spanish, makes a noun refer to a nonspecific item or items of its class.
In English, there are only two indefinite articles, "a" and "an." In Spanish, there are four indefinite articles, un, una, unos, and unas.
Spanish and English have different grammatical rules concerning when indefinite articles are needed or should be omitted.
Agreement in Number or Gender Matters
In Spanish, number and gender make a difference. Is the word plural or singular? Is the word masculine or feminine? The Spanish indefinite article must agree with the gender and number of the noun that follows it.
Singular Forms of the Indefinite Article
There are two singular indefinite articles, un and una, translating to "a" or "an." Un is used when referring to a masculine word, for example, un gato, meaning, "a cat." Una is used before a feminine word, as in una persona, meaning, "a person."
Plural Forms of the Indefinite Article
There are two plural forms of indefinite articles in Spanish, unos and unas, translating to "a few" or "some." Unos is masculine. Unas is feminine. In this case, the correct form to use depends on the gender of the word being described, for example, "She is reading some books," can be translated to be Ella lee unos libros. Although a female is reading the books, the word being described is libros, which is a masculine word, therefore, the article uses the masculine form of the word.
An example of unas being used in a sentence would be, Yo sé unas palabras en español, which means, "I know a few words in Spanish."
Although the word "some" is considered an indefinite article in Spanish, the word "some" is not classified as an indefinite article in English. "Some" is considered as an indefinite pronoun or a quantifier in English.
Exceptions to the Rule
With every language, there will always be exceptions to the rule. When a feminine singular noun begins with a stressed á, a, or ha, the masculine indefinite article is used instead of the feminine indefinite article to aide in pronunciation.
For example, the word, águila, meaning, "eagle," is a feminine word. When referring to "an eagle," instead of saying una águila, which sounds clunky in pronunciation, the grammar rule allows a speaker to say un águila, which has a smoother flow. The plural form remains feminine because pronunciation is not affected when a speaker says, unas águilas.
Similarly, the Spanish word for "ax" is hacha, a feminine word. A speaker would say, un hacha, as the singular form and unas hachas as the plural form.