There are many types of Spanish adjectives. We will begin with descriptive adjectives, which are the words used to describe nouns or pronouns. They tell us the characteristics of the noun/pronoun they go with - such as: shape, color, size, or appearance.
A descriptive adjective agrees in gender (masculine, feminine) and number (singular, plural) with the noun/pronoun it modifies. Let's look at some example sentences:
Esta camisa es muy bonita.
This shirt is very pretty.
Hay muchas camisas muy bonitas.
There are a lot of very pretty shirts.
Notice that adjectives are usually placed after the noun/pronoun.
Febrero es un mes corto.
February is a short month.
Creemos que ella es bonita.
We think that she is pretty.
Es un día terrible.
It is a terrible day.
Ellos son los más feos de la escuela.
They are the ugliest in the school.
Me gustan los zapatos de tacón alto.
I like the high heel shoes.
The principal function of adjectives is to modify or otherwise describe nouns. Therefore, the number (singular or plural) and the gender (masculine or feminine) of an adjective depends on the noun involved. As you know, the gender of a noun often has nothing to do with the concept of male vs. female but is rather the result of centuries of grammatical and spelling conventions.
la casa bonita - the beautiful house
las casas bonitas - the beautiful houses
El gato es gordo, blanco y negro. - The cat is fat, white and black.
Los muchachos son rubios. - The boys are blond.
Regular adjectives (four forms: -o, -a, -os, -as).
Most adjectives have both masculine and feminine, singular and plural forms: the “masculine” vowel is -o, and the “feminine” one is -a. An -s is added to either vowel to form the plural.
un libro nuevo - a new book (masculine singular)
dos libros nuevos - two new books (masculine plural)
una mesa nueva - a new table (feminine singular)
dos mesas nuevas - two new tables (feminine plural)
Adjectives with two forms.
These adjectives usually end in an -e, in -ista, or in a consonant in the singular form; they use the same form for both masculine and feminine forms. To form the plural, add an -s if the singular ends in an unstressed vowel, or -es if it ends in a consonant or í or ú.
un libro interesante - an interesting book (singular)
dos libros interesantes - two interesting books (plural)
una obra difícil - a difficult work (singular)
dos obras difíciles - two difficult works (plural)
Adjectives with special feminine forms.
Adjectives of nationality whose masculine singular form ends in a consonant and adjectives ending in -dor have feminine forms ending in -a and -as.
un libro español* - a Spanish book (masculine singular)
dos libros españoles - two Spanish books (masculine plural)
una obra española - a Spanish work (feminine singular)
dos obras españolas - two Spanish works (feminine plural)
*Note the distinction between un libro español (= a book from Spain) and un libro de español (= a book about the Spanish language).