Translation:Did you prepare the food yet?
What is the difference between fos and go foill?
They overlap on "yet, still" when used adverbially, but both have other meanings as well.
fóill - see 3)
OK. I'll worry about the other meanings later.
Why wasn't "arbh" used? I was under the impression that Ar becomes Arbh when preceding a vowel.
Arbh is a form of the copula. The past tense interrogative ar doesn't change before a vowel.