"Have you completed the work?"
Translation:Dych chi wedi cyflawni'r gwaith?
Cyflawni = accomplish, achieve, complete,
Cwblhau = complete, finish, fulfil
Gorffen = finish,
These are the most important definitions from the dictionary.
There is overlap between Cyflawni and Cwblhau and there is overlap between Cwblhau and Gorffen.
In general use 'Cyflawni' is accomplishing/achieving and completing in the sense that it is an accomplishment; 'Cwblhau' is completing a task or some work; Gorffen is finishing something, often a meal.