"She is not here yet."
Translation:Níl sí anseo go fóill.
Could be wrong, but even though fós can be translated into English as either "still or yet", think of fós as meaning only "still" as think of go foill as meaning "yet". "Still" or fós would apply to an action that has been going on and continues to go on. (i.e, He is still working). "Yet" or "go foill" would apply to something that has not happened as of the present moment. (i.e, He has not started yet).
@chewbacca4213. Níl if the positive sentence is using Tá. Ní if the positive is using Is. Here, positive = Tá sí anseo. (There is no essential equality between her and being here, she exists in other places too) Is cailín í (she = girl) would have negative: Ní buachaill í (her not= boy)