Would "Dw i meddyg" also be correct?
No this would have to be "Dw i'n feddyg". But that is a weirdish way fo saying it since we normally use the emphatice to say that we are something.
Would the negative be, "Meddyg dw i ddim", "dim meddyg dw i", "nid meddyg dw i" o'r something else?
Dim meddyg dw i. (colloquial) or Nid meddyg ydw i. (more formal) would both work.
Diolch yn fawr iawn.
So why isn't it "Meddyg dw i'n"?
Because "yn" is only used to connect a form of "Bod" to a noun when the noun follows it.