"Do you eat meat in Abergavenny?"
Translation:Dych chi'n bwyta cig yn Y Fenni?
Wikipedia (as always!) seems to have an explanation: Apparently it used to be Aber Gafenni, but in Welsh got shortened to Y Fenni. Aber means 'river mouth', and Gafenni/Gavenny is the name of the local river.
There's some stuff about the origin of the name gafenni too. https://en.wikipedia.org/wiki/Abergavenny#Origins_of_the_town_and_its_name
Yes - Duolingo is generally not too great on teaching grammar except by example. And things that can't be distinguished clearly in English (e.g. singular/plural + formal/informal "you") are hard to teach this way.
Tips+Notes written by course maintainers plus sentence discussions or other forum posts on Duolingo can help, as can supplementing your Duolingo learning with other resources for grammar.
(And if you're using an app for Duolingo, consider using the website at least to view the Tips+Notes for each skill, which aren't available on the app.)