Translation:Ceri Lingo will not be going to the party.
This is a negative sentence - note the ddim - and the verb at the start of a negative sentence is subject to the mixed mutation. This is because of the 'ghost' of negative particle ni which is not now normally used in the colloquial language but nevertheless still causes the mutation.
In the mixed mutation, the letters p, t, c undergo aspirate mutation to ph, th, ch and the letters b, d, g, m, ll, rh undergo soft mutation. (In informal speech some people do not use the aspirate mutation and they may use the soft mutation instead.)
Because there used to be a "Mi/Fe" verbal particle there (note verbal particle this has nothing to do with the pronouns mi/fe). So "Mi/Fe fydd Ceri lingo..." would be the traditional form of this sentence. In modern Welsh the "Fe" particle is dropped in South Wales (where they used to use "Fe") so it is now correct to either mutate softly the verb or leave it unmutated. The "Mi" particle is still used in the north where I come from. As you said you must mutate softly for a question. EDIT:This is the explanation for a positive sentence, please look below to see the correct explanation for negative sentences.
I put "will not go" rather than "will not be going" and it was correct. Just for clarification, does it have both meanings? IE:
Ceri Lingo will not be going to the party --> gives the impression that there should be a reason, or that someone else has made the decision for him/her
Ceri Lingo will not go to the party --> sounds more like he/she is refusing to go
Are both of these translations correct?
Thank you :)
Either wording is a grammatically correct translation. However, there is no sense in the Welsh of her not wanting to go or being stopped from going, so the usual and preferred English equivalent is 'she won't/ will not be going'
Unfortunately Duo does not give us the option of differentiating between translations that are 'good' and ones that are 'well, yes, OK but a bit odd in this context'.