"This dog is old."

Translation: Ten pies jest stary.

[BatyaMatzk]

Why not "Ten pies to stary"?

[Jellei]

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.

[immerweiter]

why ten and not tego?

[immery]

In this sentence, This dog is a subject, so it takes a nominative case - ten pies

[JefferyCha9]

This dog is singular not plural. For the answer to be correct it'd have to say ten psy

[E_MALYY]

i forgot when we use "tę" and "ten" in which cases?

[Jellei]

https://www.clozemaster.com/blog/polish-demonstrative-pronouns/ - this explains it all.