An adjective must be aligned with respect to its genus, or gender (masculine or feminine) and with respect to its numerus, or number (singular or plural).
Most Spanish adjectives end in their natural masculine form in -o. If the noun is plural, an -s is added.
El perro blanco. Los perros blancos.
The white dog. The white dogs.
If the noun is feminine, the adjective usually ends in -a, or in the plural in -as.
La gata blanca. Las gatas blancas.
The white cat. The white cats.
When an adjective is used for feminine and masculine nouns together, it is put in its masculine plural form.
La chica y el chico son muy simpáticos.
The girl and the boy are very nice.
There are also adjectives in Spanish that have alternative endings such as on -e. In these, an -s is added only in the plural, otherwise they keep their natural form.
La chica inteligente. El hombre inteligente.
The intelligent girl. The intelligent man.
Los chicos / hombres inteligentes.
The intelligent girls / guys.
Other common adjectives end in consonants, such as -ón, -ín, -és. In these, an -a is added for feminine nouns and an -es or -esa in the plural.
Es un queso holandés. Es música holandesa.
It is a dutch cheese. It is dutch music.
Son trajes holandeses. Son mujeres holandesas.
They are dutch clothes. They are dutch women.
There are a few common Spanish adjectives that can be placed before the noun in the singular, such as the words "good" (bueno) or "bad" (malo). In the case of a masculine noun, the adjective loses the -o ending.