The current work, Dark Roads, is set in the medieval period, 1201 to be precise. How people spoke then was effectively a completely different language to what is spoken now, so much so that we would not understand someone from those times, nor they us.
Given that, how do we write dialogue that sounds sufficiently archaic but still understandable? At a pedantic level, I’m sure we can’t, the two needs are mutually exclusive. That doesn’t remove the need though, or at least I don’t think it does.
My solution for what it’s worth is to use a language that my wife describes as ‘Robin Hood’. It sounds archaic, but has little or no basis in reality. The trick, I think, is not to overdo it (or at least not by too much). So I don’t have any ‘Gadzooks’ or ‘Zounds’ in there, but the order of words is slightly diferent from modern usage and there are words used that are correct and understandable but now rather out of fashion.
Like a lot of things in this writing game, it’s a bit of a balancing act.