"Are you laughing?"
Translation:Wyt ti'n chwerthin?
I have here an unnecessarily technical grammar explanation, which could be confusing (I find it so!). I'd be grateful if somebody who knows more about these things than I do could verify!
I believe the reason is to do with the original formal grammatical conventions of Welsh. Rwy'n (a formal way of saying 'I am', like 'dw i'n') and rwyt ti'n are (I think) both contractions, of which the full structures are 'Yr wyf i yn' and 'Yr wyt ti yn' respectively (this is seen for plural forms as well; its effect on the 3rd person isn't seen in singular or plural because mae/maen[t] obviously begin with a consonant, so the particle 'y' is easier to lose). Dw i'n is also a contraction, its original form being 'Yr ydw i yn', and it can also be seen written 'rydw i'n'. (In fact, ydw and wyf are technically both different contractions of the same form, ydwyf. Welsh has too many contractions.)
All of these are affirmative forms (i.e. statements), and this is shown because all use the particle 'yr' or 'y', which here has nothing to do with the definite article 'the'. By contrast, questions do not use 'y', the "affirmative particle", but rather 'a', the "interrogative particle". As such, what was previously 'yr wyt ti yn', or in spoken Welsh, 'rwyt ti'n', now becomes 'a wyt ti yn...?'. This introduces a vowel clash as there is no consonant added to smooth pronunciation, and so this particle is lost completely in spoken language, leaving only 'wyt ti'n...?', contrasting with the R from 'Yr' which has endured in the spoken form of the affirmative verb.
I'm sorry that this has been so long-winded; it's really not a necessary level of detail for... well, anyone. The summarised version is basically: 'The Welsh verb 'to be' is made up of lots of words which have shrunk in colloquial Welsh into one word.'
I've entered this thread three years too late! But, I found it interesting and helpful. Language is fluid, profoundly dependent upon human emotion and predilection toward laziness. Habits become rules over time, but are still subject to majority rule!
I think the second law of thermodynamics (entropy) may also be in play here.