"This building is a stadium."

Translation: Ten budynek to stadion.

[Hatul_Madan]

Lepiej brzmi Ta budowla to stadion.

[Alex149868]

Why not "Ten budynek jest stadion"?

[Jellei]

It's ungrammatical. In a "[noun] is [noun]" sentence, the noun after any form of the verb "być" (to be) needs to take the Instrumental case, so "Ten budynek jest stadionem".

I often say that the variant with Instrumental is more elegant, but here... I think that the option with "to" is a lot more natural. It's not that "this building is a type of a stadium", but more of "this building = a stadium". So it's different from let's say "A cat is an animal".