Adjectives are used to describe properties of the substantive they come with. They have always the same gendre and number as the substantive.

Some have two terminations, one for femenin one for masculin:
El coche nuevo. Los coches nuevos.
La casa nueva. Las casas nuevas.

Other have one common termination for both gendres:
El coche grande. Los coche grandes.
La casa  grande. Las casas grandes.

Other examples of one-termination adjektives are:
azul - verde - naranja - violeta
blue - green - orange - violett

As an informal rule, we could say that every adjektive which does not end with "-o" is potentially a one-termination adjektive. Every adjektives which ends with "-o" is a two-termination adjektive because the "o" can be replaced with an "a" (building so their femenin form).

Building the femenin (or masculin) of the adjektive or building the plural is as easy as with substantives and the same rules are applied:

Masculin Femenin
nuevo nueva
feo fea
loco loca
grande grande

For plural you only have to add an "s" or "es" to the end. If it ends with vowel add "s" if it ends with consonant add "es".

Singular Plural
nuevo nuevos
guapa guapas
grande grandes
cruel crueles
fiel fieles

The adjektive in the sentence

In spanish the adjektive can be placed before the substantive or after it:

La casa bonita. La bonita casa.             - In singular
Las casas bonitas. Las bonitas casas.  - In plural

Vocabulary used in the examples:

casa - house
coche - car
cruel - cruel
feo , fea - ugly
fiel - fidel
grande - big
guapo, guapa - beautiful
loco , loca - crazy
nuevo, nueva - new