The difference between wnes i ddim and do’n i ddim is one that confuses almost all learners initially, and often the secret is to just say the one you think, make a mental note when it comes back differently, say it softly along with the second speaker to help get used to how it feels, and carry on. The more you hear it, the more familiar it becomes, and eventually it feels natural to you. You might not be able to explain why it’s right, but it ‘sounds’ right to you - just like a native speaker.
However, with it being a frequent question, there are quite a few threads on the forum you might find useful. Here are a few: