Learn Polish: Wersja premium jest świetna. - The premium version is great.

Because premium is a borrowed word, it is indeclinable in Polish. This means it never changes its form. No matter the gender, number, or case of the noun it modifies, it stays exactly the same: wersja premium, samochody premium, and so on.

Since we are using the verb być (to be), why is świetna in the Nominative case instead of the Instrumental case?

When using być to describe a subject with a stand-alone adjective (like saying something 'is great'), the adjective stays in the Nominative case. You only use the Instrumental case after być when it is followed by a noun, for example: Wersja premium jest świetnym produktem (The premium version is a great product).

Why does premium come after the noun (wersja premium) instead of before it?

In Polish, adjectives that classify something (telling you what specific category it belongs to) generally go after the noun. Premium tells us the category of the version. On the other hand, descriptive adjectives that just give a quality or opinion, like świetna, usually go before the noun when placed together in a phrase.