From Gerald Erichsen,
Your Guide to Spanish Language.
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As in English, nouns in Spanish can be either singular or plural. But, unlike in English, adjectives in Spanish can also be singular or plural. Also, nouns in Spanish can be either masculine or feminine, and so can adjectives.

The general rule of noun-adjective agreement in Spanish is simple: Singular nouns are accompanied by singular adjectives, and plural nouns are accompanied by plural adjectives. Masculine nouns are described by masculine adjectives, and feminine nouns are described by feminine adjectives. In other words, the adjectives chosen to describe nouns must match in both number and gender.

Generally, the "normal" form of adjectives, the form you will find listed in dictionaries, is singular and masculine. To make the adjective plural, follow one of these steps:

• If it ends in an unstressed vowel, add -s. Examples: verde ("green," singular), verdes ("green," plural). El árbol es verde, the tree is green. Los árboles son verdes, the trees are green.
• If it ends in a z, change the z to a c and add -es. Example: feliz ("happy," singular), felices ("happy," plural). Soy feliz, I'm a happy person; somos felices, we are happy people.
• If it ends in another consonant or a stressed vowel, add -es. Example: difícil ("difficult," singular), difíciles ("difficult," plural). La tarea es difícil, the task is difficult; las tareas son difíciles, the tasks are difficult.
• Note that in some cases it is necessary to add or remove an accent mark to maintain the stress on the correct syllable.

Making a masculine adjective feminine is even easier. Just follow these steps:

• If the singular masculine adjective ends in an -o, change it to an -a. Example: pequeño ("small," masculine singular), pequeña ("small," feminine singular). El gato es pequeño, the cat is small; los gatos son pequeños, the cats are small; la chica es pequeña, the girl is small; las chicas son pequeñas, the girls are small.
• If the singular masculine adjective ends in any other letter, the feminine form is the same. El autobús es grande, the bus is big; la casa es grande, the house is big.

As you will learn elsewhere, adjectives can come before or after nouns, or they can be used with verbs such as ser ("to be") to describe nouns. But (except for invariable adjectives) they will always match the nouns they describe in both number and gender.

Important note: Invariable adjectives don't change in form and are very rare. Most of them are either uncommon colors or words of foreign origin. An example is web as in la página web (the web page) and las páginas web (the web pages). As a beginner you will seldom have the need to use invariable adjectives, but you should be aware that they exist so they don't confuse you when you see them.