Postby silmeth » 2021-12-23, 23:12
You generally use it whenever the predicate is a noun phrase – you state what or whom something/someone is: Piotr jest małym dzieckiem ‘Piotr is a little child’, Poznań jest dużym miastem ‘Poznań is a big city’, Dublin jest stolicą Irlandii ‘Dublin is the capital city of Ireland’…
You use nominative when the predicate is an adjective – when you describe what someone or something is like: Piotr jest mały ‘Piotr is little’, Poznań jest duży ‘Poznań is big’.
One exception to this are sentences with the pronoun to ‘it’ as their subjects, they use nominative: to jest duży kot ‘it is a big cat’.
And the same is true when you use the X to (jest) Y copular construction: Piotr to małe dziecko ‘Piotr is a little child’.
Sometimes you’ll see different usage though, mostly for stylistic reasons: jam jest syn Tuhaj-beja! ‘I am the son of Tuhaj-bej’ in a book by Sienkiewicz with nominative. More rarely yet you’ll see describing adjectives in instrumental (but it may happen sometimes too).