As you can see, while adjectives are invariable in English, they do change in Spanish. The rule is very simple, though: Adjectives and the nouns they modify must agree in gender and number.
In Spanish, adjectives must agree with the noun (or pronoun) they describe in gender and in number. This means that if the noun an adjective describes is feminine, the adjective must be feminine, and if that same noun is also plural, the adjective will be feminine AND plural as well.
