Spanish adjectives, like nouns, have gender, singular and plural forms. The gender and number of adjectives should match those of the noun or the subject.
When modifying a noun, Spanish adjectives are usually placed after the noun, but sometimes they are not, and their meaning can change depending on whether they are placed before or after the noun.
a thick book
All three of “un”, “libro” and “grueso” are masculine singular.
a large table
All three of “una”, “gran” and “mesa” are feminine singular.
These books are old.
In the example above, we use the masculine plural adjective because the subject, “Estos libros (These books)”, is a masculine plural.