Originally posted by: ibisc https://forum.duolingo.com/comment/13844144
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'n wedi bwyta*) doesn't work - an action cannot be both incomplete and complete at the same time.