I am really confused about when to shorten ‘eu’ (their) to 'w and when to shorten to 'u.
If I want to say “and their” it’s “a’u”, but if it’s “to their” it’s “i’w”. Why does the “eu” shorten to “'w” instead of “'u”? And how do I know which to use? Do all prepositions take 'w or is it more complicated than that?
I’m afraid I don’t know, but I suggest you put this in the “Tiny questions with quick answers” thread. If you need help finding it, it’s in the General / Questions part of the forum. Lots of grammar nerds / experts look at that thread regularly so it’s usually not a long wait.
Good call. Will do that!
I think it’s pretty much because after i, it’s easier to pronounce.
The best way is just getting used to hearing them and (particularly) seeing them written down. I say that, but I still stumble even after 20 years of speaking Welsh as a second language!
When it comes to (any) grammar, it’s almost always more complicated than that! 
It’s only the i + eu that becomes 'w, but eu can become 'u or 'i too.
Have you got any of Gareth King’s grammar books? You’ll find much more definitive answers in those, but I won’t be able to look at mine for you until I’m home from work this evening. @garethrking
Thanks!
I have, I think, almost all of Gareth’s books so will have a look. I was perhaps just being a bit lazy.