"Wnest ti ddweud dy fod yn dod neu na?"

Translation: Did you say that you are coming or not?

[crush]

Why is it dy fod yn and not something like bod ti'n? Also, is neu na the usual way to say "or not"?

[EllisVaughan]

Yes neu na is the usual way of saying or not. Both Dy fod yn/dy fod di'n and bod ti'n are fine the only difference is that Dy fod yn/dy fod di'n is more traditional.