"Byl jste tady nový."

Translation: You were new here.

[ion1122]

Here "byl" is singular, so "jste" cannot be you plural; it must be you singular polite form!

[tatka_]

Yes, "byl" is singular and "jste" is formal singular (or plural). Please read Tips and notes: https://www.duolingo.com/skill/cs/To-be-plural/tips-and-notes

The sentence for plural is "Byli jste tady noví."

[JanLyko]

Exactly! Accordingly "Byla jsi / byly jste / byla jste".

[BrianSille2]

A problem with audio. I hear a distinct š or ž sound at the end which makes "nový" difficult to recognise. I have reported it.

[VladaFu]

Which voice, male or female?

[BrianSille2]

It was the female voice, but in a later exercise the "novy" was clearly audible from the same voice.

[VladaFu]

I cannot hear any problem when listening from the audio button above.