Redoing challenges from Level 1 I’m discovering details that the first time I had totally missed while desperately trying not to drown in an ocean of new words and sentences! 
One is that Iestyn says
It is = mae hi’n / mae fe’n / mae’n
But seeing the vocabulary list it says
It is = mae hi’n / mae o’n
What is mae o’n?
Second doubt.
Why yn Gymraeg but yn Nghymru? 
They both normally start with C and are preceded by yn…so what’s the difference?
‘Fo/o’ is the Gog version of ‘fe/e’.
Because it should be ‘yn y Gymraeg’ and being feminine will soft mutate after the definite article.
Thanks a lot for your answer.
Now I’m wondering, do you often have to guess there’s an imaginary article to decide for which mutation to go for, or it’s just the Cymraeg that’s a bit special?
I, and other Gogs, don’t have to guess as it’s explained by Aran in our course. 
But, normally, no.
p.s. now before accusing Iestyn of not explaining things, I’ll have to listen to that part again - cause I noticed I tend to use the explanations parts to let my brain rest and kinda ignore them. 
I just discovered quite a few things I didn’t remember hearing at all, the first time I did the challenges - so I might have missed something again!
It seemed clear yesterday, until more examples appeared and now I’m confused again.
I’ve seen or heard:
Mae’n ddiddorol
Mae’n fyd mawr
Mae’n mynd
Mae’n tynnu
Mae’n dda
Mae’n cerdded
yn Nghymru
ym Mhen Llyn
My guess for the first series is that when people speaks informally, once again, mutation doesn’t always happen?
My guess for the second set is that yn has a different meaning and therefore causes a different mutation?
(or maybe it depends on something else before or after that I can’t remember now)
Nouns and adjectives change with soft mutation after yn or 'n (diddorol, byd, da). Verbs don’t (mynd, tynnu, cerdded).
Nouns and place names change with nasal mutation after yn (in).
How do you know when to use ‘fel’ or ‘yn’ for ‘as’.
Well, yes, that is often true, but the reason mynd, tynnu and cerdded haven’t mutated is that they’re verbs, and verbs don’t mutate after this kind of ‘yn’ (the others are either adjectives or nouns, which DO mutate).
Yes, the ‘yn’ here means ‘in’ and this type of ‘yn’ causes a nasal mutation.
I’m not sure what you mean - ‘yn’ doesn’t mean ‘as’. Can you give me an example?
I’m sorry Dafydd, could you give an example of ‘yn’ used as ‘as’ please…
Ahh, ok, thanks @gruntius and @siaronjames!
I realized I thought “everything mutates when X happens” while sometimes it’s “some categories of words mutate when X happens”.
I’m sure what I just wrote is not an accurate explanation of the grammar rule but it’s clear to me now. 
no, no, noooo - you carry on! I’m supposed to be working anyway!
ond dw i ddim yn gwybod beth i’w gael yn bresant iddi.
ok, so although the English comes out as “as a present for her”, this is a ‘logical’ translation, not a ‘word-for-word’ translation. The yn here does not mean ‘as’ in Welsh, it is turning ‘presant’ into an adjective, which is just a way Welsh has that of implying an ‘as’ in this example. You could equally use ‘fel’ (which does mean ‘as’) in the same sentence to get a ‘word-for-word’ translation.
Thank you
@gisella-albertini - you’ve gotten great answers from Gruntius and Siaron, but here’s a little more. This is a scan from Gareth King’s dictionary (my very old copy that is falling apart) of the page about mutations. It is a good, concise review of the various rules and I refer to it OFTEN (which is why it is falling apart at this point). Hope it helps! (click to enlarge)
And here’s a pdf version you can download: mutations.pdf (560.6 KB)
