Gender rules in Spanish and their exceptions

Everyone who learns it knows that in Spanish nouns are marked for gender (o/a), and that they agree with their modifiers (or vice versa): for definite articles it is either "el" for masculine or "la" for feminine nouns.

But then you hear someone say "el tema" (the topic/theme). And find out that there are a number of words (which happen to come from Greek) which end in -ma which are actually masculine (sistema, programa, idioma...). But not all: gama (range, scale) is feminine.

And then there is agua. While the singular is "el agua," the plural is "las aguas." What is going on? There apparently is a phonological dissimilation rule in Spanish (not always respected, but in this case it pretty much is) which resists the coming together of a/a, like in "la agua." Try to say it, and you will find you need a glottal stop between them to pronounce both of the a's clearly. Say it fast and with no stop between the a's, and it will come out sounding like "el agua" anyway.

So far so good. Greek terms with exceptions, and phonological dissimilation aside, the system is otherwise fully regular, right? Wrong. You will hear people say both "la radio" and "el radio." But it turns out they are not talking about the same thing. "El radio" is the device (for those who still have one and do not just use Internet streaming) used to capture the radio waves, while "la radio" refers to the waves themselves, or more precisely to the programming. So "I heard the news on the (la-feminine) radio the other night and realized it was time to buy a new (masculine) radio." Here the translation into English makes it wound like the device is being referenced in both cases, but in Spanish it is clear that the first reference is to the programming. There are more of these masculine/feminine semantic distinctions as well.

And then there are cases of optional masculine/feminine gender. "La/el mar" (the ocean/sea) is one, but there are many others. Finally, there are historical versions of this last story: "el puente" means "the bridge," but there is a city in Southern California named "La Puente," for an old bridge in the area. So were the residents simply not using the correct grammatical form in this case? Rather it seems that at that time the city got its name "puente" could be either masculine or feminine (like "mar" above).

So watch out when something about a language seems regular and straightforward: there may well be surprising twists and turns in store for the unsuspecting.