Words ending in -a are usually feminine, words ending in -o are usually masculine, but it's just a common pattern or guideline. A Spanish teacher told me that every word ending in -ma is masculine; I don't know if that's literally 100% true. I don't think any language really follows rules. Spanish has some nouns that end in -a, like agua, aguilla, which take "el" instead of "la" and yet are feminine anyway. "The deep water" is "el agua profunda." "Profunda," because "agua" is feminine, but "el," just because. Spanish also has some nouns that end in -a that use BOTH "el" and "la" but mean completely different things! "La cometa" is "the kite." "El cometa" is "the comet." Now tell me: in English, why are cars, trains, planes, and spacecraft neuter ("the Challenger broke apart on its tenth flight"), but ships feminine ("the Titanic sank on her maiden voyage?") You don't notice the weird stuff so much when it's your native language.

The issue is that not all the word which end in "a" or "ma" are going to be feminine. It's true that for the most part or the majority are going to be feminine, but there are exceptions that or there are words that don't follow the rule.

Also: 1) I struggle with this _terribly_ myself. 2) I assume you've been taught "buenos días," so there's another word--dia--that is ends in -a but is masculine.

words of direct Greek origin are usually kept in Masc form. el mapa (exception) of direct Latin origin. el trauma el drama el tema el programa

There are some Spanish nouns that are sort of irregular like that. But not many. The ones that you encounter frequently you will just memorize, because it is only a handful of nouns.