"Would you not prefer to swim?"
Translation:Fasai hi ddim yn well gyda chi nofio?
The 'you' in this construction is the chi. hi here means 'it':
- Fasai hi ddim...? - Wouldn't it be...?
Literally, fasai hi ddim yn well gyda chi...? would translate as '*wouldn't it be better with you....?', but the construction actually translates into English as 'wouldn't you prefer...?