Pitman's Commercial Spanish Grammar (2nd ed.)

The Adjective in Spanish agrees with the noun it qualifies, in gender and number, as --

El periódico italiano: The Italian newspaper. La prensa americana: The American press.

It generally follows the nouns as in the above examples, although exceptions will be found, e.g., when the adjective recalls to our mind a quality which is already known to belong to it, it generally precedes the noun, as --

He visto sus hermosas flores: I have seen his beautiful flowers.[30] Adjectives form their plural in the same way as nouns.

Footnote 30: His flowers are known (or supposed) to be beautiful, before we say it.

Formation of the Feminine of Adjectives.

Adjectives ending in O change O into A, as --

El sombrero blanco: The white hat. La casaca blanca: The white coat.

Adjectives of NATIONALITY which end in a consonant add A to form the feminine, as --

El sastre francés: The French tailor.
La modista francesa: The French milliner.

Adjectives ending in N add A for the feminine, as --

Un hombre socarrón: A sly or cunning man.
Una sonrisa socarrona: A cunning smile.

EXCEPT Joven (young), Común (common), and Ruin (base, sordid), which do not change for the feminine.

Adjectives ending in or add A to form the feminine, as --

Un comerciante emprendedor: An enterprising merchant.
Una casa emprendedora: An enterprising firm.

EXCEPT Comparative adjectives ending in or, as Mejor (better), Peor (worse), which do not change for the feminine.

There are a very few adjectives ending in ete and ote. These change into eta and ota for the feminine.

Other adjectives do not change.

          Ser[31] (to be).              Estar[32] (to be).

 Pres. Part., Siendo (being)     Pres. Part.,         Estando (being)
 Past Part., Sido (been)         Past Part.,          Estado (been)

 Pres. Tense, Indic. Mood.       Pres. Tense, Indic. Mood.


 Soy (I am), etc.   Somos        Estoy (I am), etc.    Estamos
 Eres               Sois         Estás                 Estáis
 Es                 Son          Está                  Están

 Imperf. Tense, Indic. Mood      Imperf. Tense, Indic. Mood.


 Era (I was), etc.   Éramos      Estaba (I was) etc.   Estábamos
 Eras                Erais       Estabas               Estabais
 Era                 Eran        Estaba                Estaban

 Past Def. Tense, Indic. Mood.     Past Def. Tense, Indic. Mood.


 Fuí (I was), etc.  Fuimos       Estuve (I was), etc.  Estuvimos
 Fuiste             Fuisteis     Estuviste             Estuvisteis
 Fué                Fueron       Estuvo                Estuvieron

Footnote 31: Ser is used --

(1) To form the Passive voice as, Le amo (I love him), Soy amado de él (I am loved by him).

(2) To denote an inherent quality, viz., forming part and parcel with the subject, as Este hombre es hábil (this man is clever).

Footnote 32: Estar is used --

(1) To denote state in locality, viz., to be in a place, as Estoy aquí (I am here), Manchester está en Inglaterra (Manchester is in England)

(2) To denote condition (as opposed to an inherent quality), as Este hombre está cansado (this man is tired).