The verb być requires the Nominative Case only in sentences with this/that/these/those/it.
Examples
To jest książka.
This is a book.
To był dobry człowiek.
It was a good man.
Czy to są twoje buty?
Are these your shoes?
Otherwise the verb requires the Instrumental Case.
Examples
Ona jest piosenkarką.
She is a singer.
On był dobrym człowiekiem.
He was a good man.
Jesteśmy lekarzami.
We are doctors.
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