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.