Hola Dave,

The rule to follow is:

Most regular nouns that end in o are masculine.

Most regular nouns that end in a are feminine.

There are exceptions to the rule however, e.g. el problema.

This may help also:

Masculine nouns:

• Most nouns that refer to male people and animals are masculine. (un hombre, un perro, un primo, el abuelo, etc.)

• Most nouns that end in -o are generally masculine. (un gato, un globo, un cuadro, el cuaderno, etc.)

Feminine nouns:

• Most nouns that refer to female people and animals are feminine (e.g. mujer, gata, madre, prima, abuela, etc.).

• Most nouns that end in -a are generally feminine. (una cama, una casa, la salida, la mesa, etc.) 

• The majority of words that end in -dad or -ión are also feminine. (la verdad, la igualdad, la estación, la revolución, etc.).

If you type 'Gender of Nouns' into the Kwiziq search bar, you will find lots of information to help you.

Saludos

Clara