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 