Why not "Ten pies to stary"?
You cannot use "X to Y" construction is Y is an adjective. It's like saying "This dog = old".
https://www.duolingo.com/comment/16373167 - more info here.
why ten and not tego?
In this sentence, This dog is a subject, so it takes a nominative case - ten pies
This dog is singular not plural. For the answer to be correct it'd have to say ten psy
i forgot when we use "tę" and "ten" in which cases?
https://www.clozemaster.com/blog/polish-demonstrative-pronouns/ - this explains it all.