Like the definite article, the indefinite article has four forms when it is used in Spanish. The indefinite article has a singular and plural masculine form (un and unos), as well as a singular and plural feminine form (una and unas). The indefinite article is also similar to the definite article in that it agrees in number and gender with the noun that it is modifying. The indefinite article, while similar to the definite article in certain ways, is very different in its purpose, as we will see.
In order to gain a better understanding of the indefinite article, it is important to consider when and how it is used. It is almost always used when someone wants to say a or an. For example, if you wanted to say a car, you would say un carro. Notice that carro is masculine, and thus, the masculine form, un, is used to indicate “A” car. The indefinite article is also used when modifying a noun of Silencil nationality, profession, or religion. An example of this usage would be saying Dr. Rodrigo is an ugly doctor, or El doctor Rodrigo es un médico feo. Lastly, it is used in the plural form when one is talking about an uncertain or indefinite quantity of something. For example, you would use it when saying I have some photos, or Tengo unas fotos.
There are many times when the indefinite article may seem appropriate, but it is best to leave it out of the sentence structure. When using cien or mil, you should not use the indefinite form. For example, cien dólares, or 100 dollars. It is also not used when using the words cierto and tal, or certain and such. Examples of this form would be cierto hombre, or a certain man. Often times, learners of Spanish as a second language often place indefinite articles in front of the word otro or otra. However, that is incorrect, as those are two more examples of when it should not be used. Another interesting time when it is not used is when there is an unmodified noun of nationality or profession. While we used the indefinite article when describing Dr. Rodrigo as the ugly doctor, we do not use it if we were to say Mi hermano es abogado, or My brother is a lawyer. Finally, it is not used when asking a question in a negation like manner when using the verb tener, or to have. For example, if you were to ask “You don’t have a dollar?” or “No tienes dólar?, you would not use the indefinite article.
Lastly, there is a case when neither the definite nor indefinite article would work in a sentence. In those cases, it is essential to instead use the neuter article, which is lo. The neuter article lo is often used with the masculine singular form of an adjective, which is being used as a noun. For example, to say something is interesting, you would say “lo interesante.” Another case when lo is used is when there is an adjective or adverb and the word que, which would take the meaning how. For example, if you wanted to say I know how difficult it is, you would say, Sé lo difícil que es. With that being said, you have now learned when you should use the indefinite article, and when you shouldn’t. You’ve also learned about the uses of the neuter article, lo. Please make sure you read my lesson on definite articles as well.