Gender of nouns in Spanish I the rule of the ending "o" and "a".

Nouns ("sustantivos" in Spanish) are the words we use to name living beings (people, animals or plants), things, ideas and feelings. 

In Spanish, all nouns are either masculine or feminine. This is called their gender.

Whenever we are using a noun, we need to know whether it is masculine or feminine as this affects the form of other words used with it, such as  adjectives that describe it or articles  that go before it.

A first rule to identify the gender of nouns refers to their ending. Although there are many exceptions, we can say that, in general, masculine nouns end with the vowel "o" and feminine nouns end with the vowel "a". 

Some masculine nouns are, for example; techo (roof), auto (car), ojo (eye), miedo (fear), camino (road), foso (pit), horno (oven), inodoro (toilet), barco (boat). 

"Las maniobras de los bomberos en el techo del restaurante incendiado en José Ignacio" (The maneuvers of the firefighters on the roof of the burned restaurant in José Ignacio)

On the other hand, some feminine nouns are; ballena (whale), botella (bottle), alegría (joy), tristeza (sadness), boca (mouth), comida (food), mesa (table), silla (chair). 

"Cataluña limita la comida menos saludable en las máquinas, cafeterías y comedores de centros sanitarios" (Catalonia limits the less healthy food in the machines, cafeterias and canteens of health centers).

Although this rule is useful as a guide, there are many exceptions. These will be examined in another post.