Opposites are very important in Spanish - just as they are in English! Many opposites are adjectives! Adjectives describe what a noun is like, so they must agree in number and gender with the nouns to which they are attached. Adjectives that end in vowels will end in 'a' to agree with feminine nouns or 'o' to agree with masculine nouns. As an example, let's look at the opposites guapo - handsome and feo - ugly

un hombre guapo - a handsome man
una mujer guapa - a good looking woman

un hombre feo - an ugly man
una mujer fea - an ugly woman

Let's look at some example sentences:

El niño es guapo.
The boy is handsome.
El bebé es feo.
The baby is ugly.

La mesa es larga.
The table is long.
La mesa es muy corta.
The table is very short.

El libro es interesante.
The book is interesting.
La película es aburrida.
The movie is boring.

Note! Not ALL opposites are adjectives! Let's look at some example sentences:

La casa está a la izquierda.
The house is on the left.
El carro está a la derecha.
The car is on the right.

La tienda está cerca.
The store is close.
La escuela está lejos.
The school is far.

Es temprano.
It is early.
Es tarde.
It is late.

El servicio aquí es rápido.
The service here is fast.
El servicio aquí es muy lento.
The service here is very slow.