"You aren't drinking beer" wouldn't work here, as well?
No, because it’s past tense. Hukunywi bia is present tense. Technically, the past tense should be ”hukukunywa” but the two ku’s become merged.
Ah, I see. I missed the suffix. Thanks!
You aren't drinking / you don't drink beer. = Hunywi bia
You didn't drink beer. = Hukunywa bia.