Translation:D'fhág sé mo pháiste ag an bpáirc.
What is the difference in meaning between "fág" and "imigh"? I wrote "d'imigh sé", but it got marked wrong. I thought both meant "leave."
How can it be correct to say either "phairc" or "bpairc"? Is one preferred over the other in native speech?
It's a dialectal issue. I believe the ulster dialect lenites where the others eclipse
This annoys me - I regularly forget that I need eclipsis after ag an, and type ag an pháirc, but Duolingo marks it right, even though I don't use the Ulster forms anywhere else, so it should be marking me wrong!
But them someone who wants to use Ulster Irish would give out. How can Duolingo read your mind and decide you should never use Ulster?
I'm well aware of the dilemma - there is no perfect solution. I'm getting better with remembering to use eclipsis after certain prepositions, but I think I would have made faster progress if I had been corrected more often.
Try this, it could help.
Write a short story, where you will aim at using the main prepositions (ar, ag, i, ó, faoi, le) in all the combinations you can manage to fit it. Make sure the nouns you are using in the story cover all the initial letters that can be impacted, and some key exceptions or alternatives. Learn it by heart, record yourself and then use the recording to dictate it back to yourself, re-write it from memory. This is the principle of the French "dictée" that school children hate but parents love! It is an auto-dictée.
It might involve girls jumping over fences in the park, boys hiding under a table in the house, cats being lost in a park and found again under the table, etc.
Try to make it as short and condensed as possible.
Correcting yourself is the best way, as you are always at hand to do it.
Alternative: pick one article from http://www.rte.ie/news/nuacht/ a day, print it, and highlight the effect of prepositions. Try to spot "mistakes"! Share if you are in doubt.