Am I right in that the Nominative can be used only when the subject is a pronoun, otherwise it has to be Instrumental? Or is there some other rule?
Or maybe Instrumental is used to stress the subject (’Your mother is so nice! Is she a teacher?’) and Nominative in the other cases ("Je tvoje matka učitelka?" ’Is it the teacher who your mother is?’) Or maybe Nominative stresses belonging to a group?
I am not sure what you're trying to say. :/
I am not aware of any differences in usage or meaning between Je tvoje matka učitelka? and Je tvoje matka učitelkou? The second one sounds a bit more old-fashioned, though.
Thanks for your answer :)
I'm just comparing my native Polish to closely-related Czech language, trying to figure out similarities and differences. As you know, here you'd always use the Instrumental construction, except if introducing yourself by your name (while there is an idiomatic usage of instrumental in this situation, sort of 'epic reveal'; there is also another idiomatic usage, of Nominative in calling things and yourself by something other than their names).
That's why I was wondering whether there would be a difference in usage of both these constructions but as there's almost no - good to know :) That means I can report all sentences which don't allow one of them?