I saw a poster with this text:
So wären es Träume nur gewesen?
The subject seems to be "es", not "Träume". Why is the Konjunktiv plural? I'd write:
So wäre es Träume nur gewesen?
The "real" subject in this sentence is "die Träume". The pronoun "es" is just a "dummy" word. It's easier to understand if you simplify the sentence and change the word order:
This structure is called "Prädikatsnominativ", "Gleichsetzungsnominativ" or "Nominativobjekt" in German grammar. You can only use it with following verbs:
Finally I can answer again:
It is a quotation from Schillers "Don Carlos" in ancient German (follow the link for the passage).
The "Konjunktiv" is nowadays used in indirect speech to disassociate from a message (like the anchorman from the news).
To Topic: You have to know the context -- always in German, the context is important. The passage from Schillers Don Carlos:
Es for Zeiten & former ideals and actions of Carlos.
From Schiller's sämmtliche Werke in Einem Bande, Cotta 1834, p 249. the passage in an ancient font: