If the adjective ends in -o, we create the feminine form by replacing the ending -o with -a: bueno - buena, malo - mala, negro - negra.
Adjectives with a non -o ending usually do not change in the feminine gender: fiel - fiel, gris - gris.
Exception: the ending -a is taken by adjectives ending in consonants denoting nationality, as well as adjectives ending in -án, -ón, -ín, -or: inglés - inglesa, español - española, comilón - comilona. In the case of words in -és, -án, -ón, -ín the word loses its graphical accent in the feminine form. Also, adjectives ending in -dor usually add -a in feminine (encantador - encantadora).