O Adjectives

Adjectives that end in –o in the masculine singular form make up the majority of adjectives in Spanish. The -o may change to –a, -as, or -os, depending on the gender and number of the noun it is modifying. For example: 

Masculine singular:

Tengo un gato negro. 
I have a black cat.

Feminine singular:

Tengo una falda negra. 
I have a black skirt.

Masculine plural:

Tengo dos gatos negros. 
I have two black cats.

Feminine plural:

Tengo tres faldas negras. 
I have three black skirts.