Rationale for "se" in "érase una vez"
I know that érase una vez is the Spanish equivalent of "once upon a time," but I was wondering if anyone could explain the phrase in a bit more detail for me, specifically the érase bit. What is the se doing there? Is this an old-fashioned grammatical construction? If so, was there also serse, because era is a form of ser? What was the rule governing the usage of se in such contexts? When did this drop out of everyday usage? Thanks.
5 Answers
It is an archaic construction used only to start tales, along with "Había una vez..." The verb "ser" normally is simply copulative, i.e. it makes two things equal or assigns to one the property of the other (e.g. Javier es médico), but it can also mean "to exist" in some cases. In theory, something like "Era un príncipe..." could literally mean "There existed a princes...", but most people will interpret that "ser" as copulative, so they will be expecting you to complete the sentence, i.e. "Un principe IS.... what?" Remember that Spanish allows subjects and objects to appear at different locations. If a native speaker assumes that "ser" requires either a subject or an attribute, but none appears (since "ser" has no attribute when it means "to exist"), it will be tremendously confusing for this native speaker, to the point that he will conclude that the sentence is just incomplete or wrong.
But 'se' comes to the rescue! This pronoun allows required or expected parts of the sentence to be completely dropped, which is exactly what we want here: to tell others that we are not going to provide an attribute, because we are going to use "ser" with just the subject of the sentence; nothing more. A similar thing happens when you say "Se ha roto la tele": you don't need to include someone to break it. In a nutshell, that 'se' warns you that the usage of "ser" is not the one you normally expect, but a special one.
Hello I'm mexican, Thanks for the interest in the spanish, I realy apreciat it I'm not good at all in english but i like this theme, i will try to help 
Es: Present be.
Era: Past was.
Érase: Past in a supposition context.
Érase is an affirmation of something we know is only a supposition. It is not real, only a theory or a fictional history.
Suppose there was once, a far far away kingdom. Supongamos que había una vez, un reino muy muy lejano.
In spanish we can say this knowing this is fictional and a supposition but saying this like an affirmation
Érase una vez, un reino muy muy lejano.
If we say:
había una vez, un reino muy muy lejano.
We are saying this kingdom was realy and now is not exist anymore. This was not a supposition, but using Érase, we are saying it like a supposition.
I have another exemple but i don't know how to translate to english x.x
For exemple: If we have a theory when something wrong is happening and this resluted to be true.
Then we say:
¡Era de esperarse!
or
Érase de esperar que así fuera...
Like saying, i supposed that it would be the result.
Anybody else know what this means?
I also wonder.. 
érase okay, I can kind of guess its basic grammatical structure, but I do believe it's probably old fashioned as you say. It's a combination of era and se, and I like to think that the se is like the "upon" part in a way. I've just never questioned it until now because there are just some rules in spanish that are a headache to question.
