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

The noun in Spanish has only two genders.

Nouns denoting male beings are Masculine.
 "       "     female  "    "  Feminine.

When there is no real gender, nouns are called masculine or feminine according to their terminations.

GENERAL RULE. -- All nouns that terminate in a, ión, d, also abstract nouns ending in ez, are feminine, as --

La villa: The town. La nación[21]: The nation.
La ciudad: The city or large town. La honradez: Honesty.
The other terminations are masculine.

There are many exceptions to the above rule, principally in nouns ending in E.[22]

Footnote 21: English words ending in "tion" end in ción in Spanish. (Note the accent.)

Footnote 22: Appendix I -- for reference only at this stage.

Formation of the Plural of Nouns.

Nouns ending in a vowel which is not stressed and also nouns ending in E bearing the stress, add S to form the plural.

All others add Es.

EXCEPTIONS --

1. Nouns ending in Z in the sing. change it into Ces.
2. Nouns ending in S do not change unless the last syllable is stressed.

EXAMPLES --

El amo: The master. Los amos: The masters.
El cuñado: The brother-in-law. Los cuñados: The brothers-in-law.
La cuñada: The sister-in-law. Las cuñadas: The sisters-in-law.
El yerno: The son-in-law. Los yernos: The sons-in-law.
La nuera: The daughter-in-law. Las nueras: The daughters-in-law.
El árbol: The tree. Los árboles: The trees.
El examen: The examination. Los exámenes: The examinations.
El lápiz: The pencil. Los lápices: The pencils.
El ómnibus: The omnibus. Los ómnibus: The omnibuses.
El jabalí: The boar. Los jabalíes: The boars.

3. Nouns ending in Y add Es, as --

El rey: The king. Los reyes: The kings.
La ley: The law. Las leyes: The laws.

4. The following add only S to form the plural --

Papá, Mamá, Bajá (a pasha), Sofá (sofa).[23]

Footnote 23: Words ending in accented vowels are very few and the rule for those ending in á, ó, ú is not very strict.

Compound Nouns form their plural according to sense, as --

Hijodalgo (a gentleman by birth), literally, "son of something"; Plu., Hijosdalgo (literally, "sons of something ").
Ferrocarril (railway), literally, "iron railroad"; Plu., Ferrocarriles (literally, "iron railroads").

Such nouns are rare.[24]

Footnote 24: A verb as a component part does not change, as El portabandera (the standard-bearer), Los portabanderas (the standard-bearers).

 Haber (to have), auxiliary.[25]   Tener (to have, to possess).

Footnote 25: Viz. used only before the past participle of another verb, as (Yo) he hablado (I have spoken).

Pres. Part., Habiendo.            Pres. Part., Teniendo.
Past Part., Habido.               Past Part., Tenido.

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

He (I have), etc.                 Tengo (I have, I possess), etc.
Has                               Tienes
Ha                                Tiene
Hemos                             Tenemos
Habéis                            Tenéis
Han                               Tienen

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

Había (I had), etc.               Tenía (I had, I possessed, etc.)
Habías                            Tenías
Había                             Tenía
Habíamos                          Teníamos
Habíais                           Teníais
Habían                            Tenían

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

Hube (I had), etc.                Tuve (I had, I possessed), etc.
Hubiste                           Tuviste
Hubo                              Tuvo
Hubimos                           Tuvimos
Hubisteis                         Tuvisteis
Hubieron                          Tuvieron