Кем instead of кто? Why is the instrumental case required here?
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Кем instead of кто? Why is the instrumental case required here?
Быть requires the use of the instrumental case.
Быть requires instrumental case: быть врачом, быть пожарным, быть хорошим, сильным
Here: были … кем?
Both forms are ok.
Кем мы были – sounds more about job, occupation.
Кто мы были – sounds more fundamental (people, gods, ghosts).
It’s the “instrumental of state”, used with verbs like *быть* and *стать* when talking about mutable states. Strictly speaking, *быть* doesn’t universally require it, but it’s natural to use it here because life is a transformative process. You aren’t born with a lengthy career behind you; you become things and achieve things over the course of your life.
Мы is the subject here, and as others have said, быть calls for instrumental case. It doesn’t have a good direct translation into English but think of it as something like “as whom” or “as what.”