"I have not washed the potatoes."
Translation:Dw i ddim wedi golchi'r tatws.
Golchi is to wash something, ymolchi is to wash yourself. eg dw i'n golchi'r tatws = I am washing the potatoes, Dw i'n ymolchi = I am washing (myself).
The ym- prefix usually means that the action is being done to yourself:
- Dw i'n golchi'r llestri - I am washing the dishes
- Dw i'n golchi fy wyneb - I am washing my face
- Dw i'n ymolchi - I am washing (myself), I am having a wash
- golchi - to wash (something else)
- ymolchi - to wash. (implied: yourself)
I don't think you can add an object to ymolchi because it's sort of reflexive - perhaps you can "wash yourself your hands", but not "wash your potatoes" as they aren't part of yourself.