In modern Russian you could say:
- Это дерево – дуб.
- Это – дерево.
In older times you might have used сей in its neuter form for the first sentence:
- Сие дерево – дуб.
But what was used for the second sentence? Would it be сие or would it be сё?
- ? Сие – дерево.
- ? Сё – дерево.
On the one hand, there are expressions like:
- то да сё (то и сё, то-сё)
- ни то ни сё (declined like: ни с того ни с сего, ни тем ни сем)
But I get the impression that сё is only used as a rhyming pair with то in the above idioms, and that for other normal uses the pronoun would be сие. Is this correct?
For instance in ruscorpora.ru there are phrases like:
- Понятия не имею, что сие означает
- Сие было правдой
- одновременно с сим
- вместе с сим
- И с сим встала и вышла
(the declensions differ only in the nominative and instrumental: сё/сем versus сие/сим)
Or is it possible that сие and сё were both valid and were a contrasting pair with the same usages as это and то? Was сё ever used on its own, rather than just in a couple of fixed expressions involving то?