I’ve just started course 1, I’m at lesson 9, and I have a small question
Is “Dyw hi ddim yn” supposed to sound exactly like “Dw i ddim yn”?
Because while I guess I should pronounce it differently (something like ‘deew-he-thim-in’ probably), I can’t hear that on the audio of the course.
Hi - no it’s not pronounced exactly the same. More like ‘deeoo he thim un’ as against ‘dooee thim un’. Pob lwc.
“h” is often “eaten” in “hi” in speach especially if speach is fast. Don’t mind. You can say as taught - “Dw hi thim yn”
The initial H is often dropped in South Walian, especially in the Valleys Gwentian than Iestyn speaks, so hi sounds like i. Note that this also changes the relationship that hi has with a preceding a, â and gyda because it now starts with a vowel sound, so I took her becomes es i ag 'i rather than es i â hi, for example.
Whether you decide to drop your Hs is, of course, entirely up to you and not something to worry about unduly! No-one will bat an eyelid either way.
As a related aside, Valleys Welsh speakers also tend to drop their Chs, so, for example chwarae becomes warae.
Phew! You have answered something that had been worrying me.
I kept hearing “warae” (or thinking that’s what I’d heard) on Radio Cymru, and wondered if my ears were playing tricks, or if this was some sort of advanced (or literary?) mutation that I had not come across before.
Diolch!
Croeso! 
Thanks a lot to everyone. I see Iestyn keeps saying ‘don’t worry’ for a reason
I’ve just finished Gwers 9 and 10, and I must say these have been the hardest for me in terms of pronunciation. I have been agonizing over how I’m saying “dyw” and “yw”. I’m sure I have the most horrendous American accent, but I really am trying to get the pronunciation right…and these are just not like any English vowel sounds. I think my “Dyw hi ddim yn” is phonetically (English) “dew hee them un”. However, I’m using my Iestyn mantra and I am not worrying. I know if I keep practicing the accent will get better, because it already has in other ways. I can roll “gallu” and “llaeth” out so much better lately, and I feel like my speed and cadence are improving. We just need to keep on practicing 
Great job! “llaeth” is still my nightmare … “ll” in this word is once more like Spanish “s” then it becomes more inbetween “sh” and “ss” … but I just don’t worry about this anymore. I roll out and then, what happens, happens.
“Gallu” this is one of those greatest with “ll”. I love this word. It has a great and positive meaning and is quite easy to say.
“Dyw hi ddym yn” is easy for me as Slovene language has approximately (apart from ll) the same sounds and this for it’s easy to cope with those things. Even ch is quite easy to say. Surely easier then “ll”
Regarding “Dyw hi ddim yn” I rather don’t leave that “h” out no matter if Iestyn does it (but Cat is not) .
I don’t think it’s just the Valleys Welsh that do that. I grew up in Carmarthen and I can remember hearing ‘warae teg’ or ‘warae teg, mun’ quite frequently.
Well, there you are then, ware teg.