Actually, for what I have understood, when you mean something is something and you decide to say it in the "to jest x " form, the noun remains the same, you don't change it at all.
To jest walka To jest kot To jest pies To jest kobieta
I believe it works like english fixed rules like... To be used to ___ (always in the ing).
Why does the female voice pronounce kot with a "d" as in kod??? This is frustrating when in other parts of this program you are asked to" type what you hear" and often it is the same situation:: You make errors because what is spoken/pronounced is NOT the way it is supposed to be written. So shouldn't those that are speaking the language pronounce properly??