"Twenty-five years ago, Czechia and Slovakia were one country."
Translation:Před dvaceti pěti lety byly Česko a Slovensko jednou zemí.
Why is it byly? Česko and Slovensko are both neuter so I would think neuter plural byla.
"In other cases (no masculine animate component, and at least one component which is not neuter plural), the whole compound is feminine/masculine inanimate plural (the feminine and the masculine inanimate forms of verbs and adjectives are identical in the plural)." Section "Gender and number of compound phrases" at https://en.m.wikipedia.org/wiki/Czech_declension
It is really unclear for me why we have said jednou zemi? while we could say jedna zeme? is there some reason to use Instrumental here?
You can say "jedna země". But when somebody is something you can equally use instrumental.
He is a doctor. = On je doktor. On je doktorem.
She was a teacher = Ona byl učitelka. Ona byla učitelkou.
Either or is correct.