I don't understand those demonstratives... We learnt Bony/Boni with the word Azantys(sy). Why is it Bonys in this sentence ?
"Bony" and "Boni" are used as subjects of a sentence and can be translated with "this one". They are their own part of the sentence. "Bisy azantys issa" translates to "This one is a knight". Note that "Bisy" doesn't describe azantys in this sentence, it is not an adjective.
But you can also use them as adjective. For example, "bonys azantys" means "that knight". "Bonys" is not its own part in this sentence anymore, and not the subject, but just an adjective that describes "azantys". Therefore it has to be declined together with the noun it belongs to.