I am confused. Does this mean 'all of you open your one collective mouth' or 'all of you open your (personal) mouths'?
I am not sure if I understand you but "usta" if singular and plural form, depending on context. Like "drzwi", "spodnie", "nożyczki".
The English translation needs to be "open your mouths" as "otwórzcie" refers to multiple people. For one person, it would just be "otwórz usta", wouldn't it?
You're right. I'll remove any plural options, as we are translating singular "mouth" here.
It's a childish way of saying this, but I think it's common enough to add it.
why here not accusative
It is. But "usta" are 'not masculine-personal plural', so the Accusative form is identical to Nominative.
Pretty rude. Interesting that you're the second person to ask that recently...