"The shop is a dump."
Translation:Mae'r siop yn dwll o le.
EllisV has it right.
The thing is that you cannot use both mae and ydy together like that - they both mean the same thing, 'is' - it is just that which one you use depends on the overall purpose and shape of the sentence.
If you are using the emphatic shape, as we have to in identifying people with their names or occupations, or as we can choose to do in other situations. then we have to use ydy/yw:
- Dewi yw e. Athro yw e. - He's Dewi. He is a teacher.
- Volvo ydy car Dewi, nid Alfa - Dewi's car is a Volvo, not an Alfa