"What is your favourite drink?"
Translation:Beth yw dy hoff ddiod?
Hoff only causes a soft mutation in nouns.
Edit: Ignore this, mizinamo is right; I was thinking of beth as in "what", not "thing" - my bad.
OK, so P question words, like pa and pwy. Does it ever appear unlenited?
No, it's a coincidence that it has the same letter as the question words.
peth means "thing" and is related to "piece", i.e. it always had a p-, rather than evolving from kw-/q- like the question words.
beth "what?" may have come from pa beth "which thing?" and acquired its permanent lenition from the leniting property of pa "which?".
So, it never appears as peth in the meaning what, only in the meaning thing? If that is the case, the pa beth hypothesis sounds pretty convincing to me.
Not part of the P-Celtic/Q-Celtic definition, eh? Oh, well, I prefer to think of them as Celtic-I-can-pronounce-by-looking-at-the-written-word Celtic and Jumble-of-letters Celtic anyway.