"Naddo, es i ddim i'r siop ddydd Mercher."

Translation: No, I did not go to the shop on Wednesday.

[Malwen]

Why is "dydd" mutated here?

[mizinamo]

To show that it is "on Wednesday" rather than simply "Wednesday".

I believe it's short for "ar ddydd Mercher" (on Wednesday), where the "ar" (on) dropped out in casual speech but its associated mutation remains behind.

[aadambialas]

Compared to irish the whole mutation system in welsh is much more irregular, not to say chaotic, at least for a beginner...

[Malwen]

Diolch

[aadambialas]

What is "naddo" here?

[mizinamo]

"No", for answering a question in the simple past tense. ("Yes" would be "do" in this situation.)