"After Dewi Lingo went, I had a coffee."
Translation:Ar ôl i Dewi Lingo fynd, ges i goffi.
The base form (un-mutated) is used. I wouldn't recommend thinking about them as all acceptable spellings and instead grammatical variant of the original word. E.g gath, nghath, and chath would never be seen in a spelling test because they technically aren't words in their own right instead they're variants of Cath.