‘Do, nes i ei fwynhau’ - what is that ‘ei’ please?
‘Dw i isio ei ddarllen yn fawr iawn’ - there it is again, and triggering a SM. Why?
And you’ve not only chucked ‘ydw/yndw’ at us with no explanation, but also ‘nac ydw/dw’. (and ‘do’, but I think I get that one.)
I know you’ll say ‘don’t worry about it’ - but I do!
I am enjoying the course, am making great progress, am very happy, and even survived 13 and 14 without trauma, so this is a very minor moan.
But this is a bit naughty!
Hwyl
The ‘ei’ in these is the ‘it’ -
‘Do, nes i ei fwynhau’ = Yes, I enjoyed it
‘Dw i isio ei ddarllen yn fawr iawn’ = I very much want to read it
The mutation is triggered if the ‘it’ is referring to a masculine noun.
The thing is though, there are slight variations on how to say ‘it’ in Welsh - this is not the only way, but rather than confuse things with all the variations now, the best thing is to either ask about them when they crop up, or (if you really want to find out about them now) you could use the search facility, as this has cropped up before, usually in the enormous Tiny questions with quick answers - continuing thread thread (but you’ll need to search with more words than just ‘ei’) - here’s one thread to start you off Two questions about "it" in Challenge 16
As for chucking things in without explanation, yes, it’s a bit cheeky but it’s a key part of this learning method. It mimics the fact that in normal conversations you will often be thrown words you haven’t come across and so it gets your brain used to recognising new words and fitting them into the context, but they will be repeated again in the course, so everything will fit into place eventually.
If you haven’t guessed them yet, ydw/yndw = yes I am and nac ydw/dw = no I’m not 
Thanks for the quick reply, and it answers my questions/moans nicely! You also pre-empted my next one, which would have been: why did we then get 'mae hi isio ei gwylio ‘n fawr iawn’ without the SM a little later? Presumably because we’re talking about a film now, so feminine, and no mutation?
Yup, spot on!
Of course, you’ll come across situations where you don’t yet know whether the ‘it’ is masculine or feminine. In these cases, and you know I’m going to say this… don’t worry! It really won’t matter at this stage if you guess the wrong one! Either go with your gut instinct (you may well have subconsciously absorbed which it should be and whether it sounds right) or, because there are generally more masculine nouns than female ones, go with the masculine. But overall, don’t overthink it 
Diolch yn fawr iawn!