Three uses of 'yn' and one of 'wedi'...
In Welsh, yn pops up in different guises and one of them can get mixed up with wedi, so...
Literally, yn means 'in', so we might think of its three usages like this [Caution! - the first and second outlines are simply aids to memory, and they are not 'grammatically correct' explanations]:
Firstly (yn + verb-noun, with no mutation, not translated into English):
- Dw i'n bwyta as 'I am (in the unfinished process of) eating' - I am eating, I eat
- Ro'n i'n bwyta as 'I was (in the unfinished process of) eating' - I was eating, I used to eat
- Bydda i'n bwyta as 'I will be (in the unfinished process of) eating' - I will be eating, I will eat
Secondly (yn + noun or adjective, with weak soft mutation (no effect on ll or rh), not translated into English):
- Dw i'n flewog as 'I am (in the state of being) hairy' - I am hairy
- Hoffwn i fod yn feiciwr blewog - 'I would like to be (in the state of being) a hairy biker' - I would like to be a hairy biker.
And thirdly (yn/ym/yng + noun, with nasal mutation, translated as 'in') as a preposition showing where:
- Mae Dave a Si yn y gegin. - Dave and Si are in the kitchen.
- Dw i yng Nghaerdydd. - I am in Cardiff.
- Gwnaeth Dave goginio yng Ngelli ddoe. - Dave cooked in Gelli yesterday.
- Gwnaeth Si goginio ym Mangor ddoe. - Si cooked in Bangor yesterday.
- Note a common exception - Mae hi'n hoffi darllen yn Gymraeg. (She likes reading in Welsh.) because yn Gymraeg is a shortened form of yn y Gymraeg where the y is dropped in colloquial usage.
(The abbreviated form 'n occurs only in the first two usages above, not when yn is used as a preposition.)
Going back to the first usage of yn above, wedi can be outlined in similar terms. Where yn + verb-noun has the meaning of an unfinished or continuing action marker, wedi + verb-noun has the meaning of a completed action marker (not translated directly into English):
- Dw i wedi bwyta as 'I am (after) eating' - I have eaten
- Ro'n i wedi bwyta as 'I was (after) eating' - I had eaten
- Bydda i wedi bwyta as 'I will be (after) eating - I will have eaten
And that is why combining wedi and yn (for example in the incorrect *dw i wedi yn bwyta) doesn't work.
That is by far and away the best explanation I have ever seen and it has taught me more than I have learned about yn/yng/ym/'n and wedi than I've learned by every other means - bendigedig a diolch yn fawr iawn! And one of the things I never knew was the name for a treiglad meddal which does not apply to 'll' ad 'rh' (a weak soft mutation). It's great to have this forum, fantastic to have Duo and the moderators and contributors are worth their weight in gold (or ils valent leur pesant en cacaouettes as my father used to say!) :-)
Just a thought, but I've seen a few questions on this in the forums, and I can't help but wonder whether it might be worth having a skill dedicated to teaching the different uses of "yn" if/when the tree is updated. It might help clear the matter up for a lot of future learners early on into their Cymraeg journey.