Artículo Indefinido - Indefinite Article

In English there are two indefinite articles:

a (or an)

In Spanish there are four indefinite articles:

Un Singular (male) a man un hombre  
Una Singular (female) a door una puerta  
Unos Plural (male) some checks unos cheques  
Unas Plural (female) some windows unas ventanas  

Remember in Spanish:
The indefinite article must agree with the gender of the noun

The indefinite article usually refers to an unspecified person or thing.

Quiero un libro - I want a book.
Tiene una idea - He has an idea.

The indefinite article can also refer to just one of something:

Hay un estudiante en la sala - There is one student in the room.

The plural indefinite article means some:

Compré unas naranjas - I bought some oranges.

When refering to a person's profession, the indefinite is not used in Spanish, although it is used in English.

Soy profesor - I am a teacher.
Ana quiere ser médica - Ana wants to be a doctor.

If unos or unas is used before an item that exists in the plural to refer to a single object (as "pants" or "glasses" in English), the article can mean "one" or "one pair":

Necesito unas tijeras. I need a pair of scissors.

Because many Spanish nouns and adjectives are not different, use of un or una indicates that the noun is meant:

Juan es cobarde John is cowardly
Juan es un cobarde John is a coward.


Using unos or unas before a number means "approximately"

El terremoto duró unos veinticinco segundos. The earthquake lasted approximately
twenty five seconds.