Indefinite articles are used to talk about the general idea of a thing, rather than a specific thing.
Tengo un perro.
I have a dog.
Un is the masculine indefinite article. The way it’s used, you really don’t know which dog the person has or what kind of dog it is.
Tengo una flor.
I have a flower.
Una is the feminine indefinite article in Spanish. In this example, the person has a flower, but you don’t know which one it is, because it hasn’t been specified.
Tengo unos perros.
I have dogs.
Unos is the indefinite article used in the plural for masculine nouns.
Tengo unas plumas.
I have pens.
Unas is the indefinite article used in the plural for feminine nouns.
Notice that in English you don’t need an indefinite article in front of plural nouns. In Spanish, however, you will need to add the article unos or unas!
Here are some more examples of indefinite articles in front of nouns:
un bolφgrafo
a pen
unos bolφgrafos
pens
una silla
a chair
unas sillas
chairs
un ßrbol
a tree
unos ßrboles
trees
To learn more about definite articles, click here: Definite Articles