Tiny questions with quick answers - continuing thread

I was making a cup of tea and thinking about Welsh, and I’ve ended up down a real rabbit hole.

“ddides ti = you said (singular)”, I thought… what would it be if I was speaking formally or to more people? I tried to look it up but couldn’t work out the stem - I looked at d(w)eud but the rules I came across didn’t see to correspond to what I knew… then I came across this thread - aha, I was mixing up “i” and “u” and it is starting to fall into place but still doesn’t quite fit the rules I found. Maybe it is just a spelling thing?

But why is it “ddudes ti” I have learnt in the challenges and not “dudes ti”? Is it because there is an elided particle there (like medrai → mi fedrai → - fedrai")? Or some other reason?

And while we’re at it, what is the plural/formal version of “dduest ti?” ddudoch chi?