Towards the end of this lesson they used ‘yn fawr’ to mean ‘very’ which I thought was ‘iawn’ because in an earlier lesson the ‘yn fawr’ added the ‘much’ to ‘very much’ (diolch yn fawr iawn).
Is there a context in which you would use ‘yn fawr’ rather than ‘iawn’ to mean ‘very’?
Also, is there a grouping in the forum for specific lesson numbers? I have searched but can’t see one so have been using General.
Yes, ‘very’ is usually ‘iawn’ and ‘yn fawr’ is usually used for ‘much/a lot’ but ‘yn fawr’ can be used as ‘very’ in the case of the past participle (although iawn can often also be used here too) -
e.g. I was very disappointed = 'roeddwn i wedi fy siomi’n fawr / 'roeddwn yn siomedig iawn
It’s really a case of just noticing which examples use which form and ‘sound right’. But even if you use the ‘wrong’ * one, it’s unlikely to be a big boo-boo!
*not ‘wrong’, just misplaced 
There’re no separate groupings for specific lessons per se - generally people add a topic in General as you have and often it turns into a longer thread when people search for help on a similar topic (so it kind of develops into a grouping organically anyway!)
Diolch yn fawr iawn, Siaron.
Thank you Siaron. The sentence is ‘I’m very surprised’ = Dw i’n synnu yn fawr.
It caught me out.