An adjective is a word that describes a noun. In Spanish, adjectives must agree with the noun they are describing in gender (masculine/feminine) and number (singular/plural).
When you look up an adjective in the dictionary, it is always in the masculine singular form. In other words, it always ends in -o . Adjectives that end in -o have four possible endings, one each for masculine, feminine, singular, and plural. These types of adjectives make up the majority of adjectives in Spanish.
| Singular | Plural | |
| Masculine | -o | -os |
| Feminine | -a | -as |
The following examples demonstrate how to make an adjective that ends in -o agree with a noun.