"After you have a shower, you have coffee."
Translation:Ar ôl i chi gael cawod, dych chi'n cael coffi.
Why did you input an incorrect sentence, then?
As for the technical reason why it was accepted: it's easier for a course contributor to say that an acceptable version is "Ar ôl i [ti/chi/chdi] gael cawod, [dych chi/dach chi/wyt ti]'n cael coffi" or the like with alternatives, rather than making two separate sentences, one for "chi" and one for "ti" to make sure that the two parts will use the same form. (Though doing so would be "cleaner".)