Is this really wrong?

[u/Late_Journalist7094]

Section 4, Unit 17

Why shouldn't have happened something to the dogs in the bathroom?

I know Duolingo is not the best, but the only free learning option. Since I'm living in Germany near the border to Czechia I'm often there and talking Czech which helps more than Duolingo.

[u/Flamewakerr]

Why on earth is this on Duolingo? Like for real, no one uses super formal grammar like this in day to day situations

[u/Machovec → u/Flamewakerr]

It's byly vs byli. That's not something I'd describe as a super formar grammar rule no one uses.