"Restaurangerna är öppna."

Translation: The restaurants are open.

[meowthecatty]

Really feels like Duolingo is mocking me rn #lockdown

[Hansel-]

ikr

[MikyNik1856]

What's the difference between "Restaurangerna är öppna." and "Restaurangerna har öppnet"?

[VendeDraug]

Is it something similar to "have opening (hours)"?

[OmegaGmaster]

What's the "ett" form of oppen?

[pH.tsx]

It is "öppet". Usually, when the "en" form ends in -n, the "ett" form will replace the -n for a -t.

You can check noun and adjective declension tables on Wiktionary:
https://en.wiktionary.org/wiki/%C3%B6ppen#Swedish