"Why didn't you guys study?"
Translation:¿Por qué ustedes no estudiaron?
Is the "ustedes" strictly needed here or can it be implied from the verb? It was marked wrong when I omitted it.
Duolingo expects the use of 'ustedes' when used with 'estudiaron'. An alternate solution is, omitting the 'ustedes': 'Por que no estudiasteis?'
To me "you guys" is not formal, perhaps another term should be used instead on ustedes
I don't understand - how to make future then, for "ellos" and this verb "estudiar"?
In Latin American Spanish, yes, "ustedes" is both formal and informal. I agree with Ruth, though—please don't use all caps when it's not necessary.
Usted is formal and very polite, tú is informal. Usted is a whole different gramatical structure. Did you know writing in all in capitals is classed as shouting and very rude? There's a lot you need to learn about being polite in both languages.