Step 1

Many opposites are adjectives!
Adjetivos (adjectives) agree in gender and number, and typically come after the noun.
An a ending usually means feminine.
An o ending usually means masculine.

opposite adjetivos

guapo - handsome
feo - ugly

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.

Es una fiesta maravillosa.
     It is a wonderful party.
Es un día terrible.
     It is a terrible day.

Note!
Even though día ends in a, it is a masculine noun, so we use an o ending for the adjective!

¡el día festivo!
      holiday!