Just so everyone will learn from my mistake - this sentence CANNOT be translated "once upon a time"
It makes me wonder how that would be translated to Russian? Раз в времья?
My understanding is that "жили-были" means "once upon a time" and is how fairy tales start. It is a fixed expression.
Yes, жить и быть fairy tails usually starts from something like жили-были дедка с бабкой
Once upon a time also can be translated as однажды. But as pointed out below in fairy tales it is жили-были
Talking about 'when something happens', accusative case is often used (ex. мы гуляли каждый день)
As it happens год is one of those words that has extra locative case separate from prepositional case. Wiktionary says you can say "в этом году" (locative) but "мы говорили об этом годе" (prepositional). Not being Russian, I can't comment on usage difference between в году and в год.
I'll just take a wild guess and say that "в" only takes the accusative (motion) and dative (location)
yep you're right, I based my guess on german ... but german doesn't have a prepositional...
I feel like a legitimate translation of this could also be "annually." Is that fair?
Seems right. If something happens more than once a year, then it becomes bi-annual, or tri-annual. However, "annually" is an adverb, while "once a year" is an adverbial phrase, so it's a question of how much license you can take with translation. With Russian/English, there seems to be lots of room for license.
In a previous exercise, I think I've seen годy instead of год - can either one be used here?