From Duolingo:

Negatives – You don’t have

• Does dim car gydag Owen. – Owen has no car; Owen doesn’t own a car.
  • Does gan Owen ddim car.
• Does ganddo fe ddim car. – He doesn’t have a car.
  • Does dim car gyda fe.
• Does gynnoch chi ddim cath.  – You don’t have a cat.
• Does dim cath gydag Olwen. – Olwen doesn’t have a cat.