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Spanish/Formation of the feminine in adjectives
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
The rules for forming the feminine of adjectives are different from those for the noun
Regular inflection of the feminine.
If a word ends in a vowel, the feminine replaces that vowel by a: el libro rojo, la casa roja. If a word ends in a consonant, the feminine adds an a: el libro inglés, la casa inglesa.
When adjectives are inflected
Adjectives are inflected or not depending on the ending of the masculine-gender form.
o: inflected. el libro nuevo/la casa nueva. Exception: el departamento piloto/la casa piloto.
a: not inflected. el amigo barsa/la amiga barsa. (Barsa is a slang term; a better example is sought.)
(*)e: never inflected. el libro verde/la casa verde.
ante: not inflected. el piojo cargante/la mosca cargante.
(*)ente: never inflected. el alumno ausente/la alumna ausente.
Stressed vowel: not inflected. El libro café/la casa café.
(*)Consonant: not inflected, but see regular exceptions below. El libro azul/la casa azul.
ín, ón: inflected. El camello bailarín/la jirafa bailarina. (*) Exception: el libro marrón/la casa marrón.
or: inflected. el cable connector/la cuerda conectora. (*) Exceptions: comparatives such as mayor, menor, mejor, peor, anterior, poterior, superior, inferior.
Adjectives of nationality ending in és are inflected: el libro portugués/la casa portuguesa.
(*) indicates that the rule is different for adjectives and for nouns. For example: el superior/la superiora, but el libro superior/la casa superior.
/Grammar