There are times in Spanish when a linking verb agrees in number not with its subject but with its predicate complement.
- Lo mejor de la televisión son los avisos.
- La mejor televisión son los avisos.
- Mi mejor amigo eres tú.
- Mis mejores amigas sois vosotras.
- ¡Esto son rebajas!
- Eso son los amigos.
It doesn’t always work this way, but when it does, this stands out notably from how concord works in English, where the verb invariably agrees with the subject alone, no matter the number of the verb.
This is really just one question, but here are my thoughts:
Is there a formal rule about this?
Does the fact that it more often seems to occur with neuter and indefinite subjects (esto, eso, lo) have anything to do with it?
Given that many other examples of it not happening are easily produced, how can one predict whether this happens or not?
Here are a few samples from searching (mostly) Google Books for instances of soy yo for instances where there’s an earlier noun in the sentence:
- El más oscuro de los hombres [...] ese soy yo.
- Tu rey soy yo.
- Los épicos, aquí el único mítico soy yo
- Un náufrago de mi propia alma soy yo
- Me importa un comino, el jefe soy yo
This very similar question about “Isto são tres perguntas” (that is, “Esto son tres preguntas”) on our sister site for Portuguese Language. As far as I can tell, whatever grammar applies here is true of both Iberian languages. That suggests to me that it must have originated in a common ancestor. There’s some evidence that it occurs in Italian. I haven’t yet found anything like it in French, but it is also possible that French lost it.