To jest, imperative or nominal
In the skill that deals with imperative, it is mentioned that after być or any other equivalent of an equal sign, the imperative is used and that after a comparison using "to" it's just a normal nominal noun. What is used in a sentence with "to jest"?
What do you mean by imperative? To jest + mianownik. To + mianownik. Do the tips show the cases in Latin or something? In this case you are simple naming things and that uses the case called "mianownik" - I guess that "nominal" is Latin for that. Seems a shame to use a second foreign language to explain Polish.
English learners who learn a language with cases usually use the English names of those cases.
Those names come from Latin, true, but "nominative" is just the English word for it - "mianownik" is a Polish word.
We also divide words up into "nouns", "verbs", "adjectives" and so on, not into "rzeczowniki", "czasowniki", and "przymiotniki".
No - Duolingo doesn't, I think, go as far as B1, or only barely.
Once students have finished the course, they can then choose to read Polish-language grammar texts if they choose and learn the required vocabulary then. (They'll have to learn tons of vocabulary after Duolingo anyway.)