The most common of these by far is uno, the number "one," which is usually translated as "a" or "an." See how it is shortened to un when it comes before a singular masculine noun: un muchacho ("a boy") but una muchacha ("a girl").
Here are seven other adjectives, all but the fifth one being quite common, that are shortened when they precede a singular masculine noun, as in these examples:
- alguno ("some"): algún lugar ("some place")
- bueno ("good"): el buen samaritano ("the good Samaritan")
- malo ("bad"): este mal hombre ("this bad man")
- ninguno ("no," "not one"): ningún perro ("no dog")
- postrero ("last"): mi postrer adiós ("my last goodbye")
- primero ("first"): primer encuentro ("first encounter")
- tercero ("third"): Tercer Mundo ("Third World")
The process of shortening these words is known by grammarians as apocopation. A few other adjectives are apocopated under other circumstances:
Grande: The singular grande is shortened to gran before a noun in both the masculine and feminine. In that position, it usually means "great": un gran momento ("a great moment"), la gran explosión ("the great explosion"). However, grande is not apocopated when following más: el más grande escape ("the greatest escape"), el más grande americano ("the greatest American").
Cualquiera: When used as an adjective, cualquiera ("any" in the sense of "whatever") drops the -a before a noun: cualquier navegador ("any browser"), cualquier nivel ("whatever level").
Ciento: The word for "one hundred" is shortened before a noun or when used as part of a number that it multiplies: cien dólares ("100 dollars"), cien millones ("100 million"). But it is not shortened within a number: ciento doce, ("112").
Santo: The title for a saint is shortened before the names of most males: San Diego ("St. James"), San Francisco ("St. Francis"). But the long form is retained if the following name begins with Do- or To-: Santo Domingo ("St. Dominic"), Santo Tomás ("St. Thomas").