Most words ending with "-a" are female.

[Lovro]

I realized that most words ending with "-a" are female. And even those not ending with "-a" tend to be male! :)
Love this language, it's pretty simple. :)

[jonathankyim]

Careful though! There are some exceptions, obviously, like "el problema"!

[Lovro]

True thx.

[Faya]

I think the exceptions result from the etymology: words (ending with -a) that are originally Greek, are male in Spanish. The corresponding Greek words are male or neuter:
["το πρόβλημα" = "to próvlima" = el problema]
["το σύστημα" = "to sýstima" = el sistema]
["ο κομήτης" = "o komítis" = el cometa]