is the negative of ydw being said as nadw / na dyw - I thought it was na gyw - unless I’m hearing it wrongly as a “d” not a “g”? 
It can be either nagw or nadw. Iestyn typically uses nagw, if I recall.
Edit: If I get a chance I will relisten to that Lesson to see if I can determine which is being used there.
ah thanks! 
Yes, Lesson 11 definitely uses nadw. 
great - thanks!