Translation:¿Qué clase empezaron ustedes ayer?
¿Qué clase ustedes empezaron ayer? invalid
Cual (with accent) no good?
That would be "Which class"
Why is "ustedes empezaron" marked as incorrect, when DL frequently use this form in a similar context?
my answer - que clase empezasteis ayer
That "vosotros" form is not used in many countries but I suppose it is correct, strictly speaking.
Yes, it is correct, particularly when the English sentence includes the very informal "you guys." Come on, Duolingo, you guys can do better than this!