Indefinite Article

English

a is used before a word beginning with a consonant
I saw a boy in the street.
(not a specific boy)
an is used before a word beginning with a vowel
I ate an apple.
(not a specific apple)

The indefinite article is used only with a singular noun; it is dropped when the noun becomes plural.  At times the word some is used to replace it, but it usually omitted.
I saw boys in the street.
I saw some boys in the street.

I ate apples.
I ate some apples.

Spanish

There are four forms of the indefinite article: two singular and two plural.

un indicates that the noun is masculine singular
un libro - a book
un muchacho - a boy

una indicates that the noun is feminine singular
una casa - a house
una muchacha - a girl

unos indicates that the noun is masculine plural
unos libros - (some) books
unos muchachos - (some) boys

unas indicates that the noun is feminine plural
unas casas - (some) houses
unas muchachas - (some) girls