Tá ceist eile agam oraibh. I know both "go fóill" and "fós" can either mean "yet" or "still" but I'm often getting confused which to use in different cases and how to translate them in certain cases.

Could you please explain when you're supposed to use one over the other and when you're not supposed to use one?

To my knowledge, as a fluent but far from perfect or native speaker, there is absolutely no difference between these two equally suitable choices.

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ЯГОН ТОҶИК НЕСТ ИНҶО???

They are mostly interchangeable words for "yet" and "still".
Just a few times that using "fóill" is preferable to "fós": Fan go fóill.

In Munster they use "fós", in Donegal "go fóill", looks like they use both in Connachta... Apart from that, they mean the same thing and are used in the same contexts, apart from what Bríd said about Connemara Irish...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Go raibh maith agaibh. cleared it up now