Translation:Twenty-five years ago, Czechia and Slovakia were one country.
Just checked, there are no tips and notes available here yet, but you can use other resources, like https://en.wikipedia.org/wiki/Czech_conjugation#Agreement_between_subject_and_predicate
but that one is still incomplete. There are links to recommended resources in one of the last reports about the course completion.
I don't know whether that is accepted or not, but here's a helpful hint for learners:
It's safer not to switch the order of clauses unless doing so results in an odd translation. Some sentences have thousands of possible translations, and unnecessary clause-switching would significantly increase that number.