This is what is written in the tips and notes: https://www.duolingo.com/skill/gd/Clothes/tips-and-notes
"This is the first time we come across a verbal noun in this course. These are similar to -ing words in English. This is a common way of forming the present tense in Gaelic. If you can use one verbal noun (you can, you've got this), then you can use any of them.
Verbal Noun 1 - ag iarraidh
- Tha mi ag iarraidh fèileadh. - I am wanting a kilt.
- Tha mi ag iarraidh IRN BRU. - I am wanting IRN BRU.
- Tha mi ag iarraidh taigeis. - I am wanting Haggis.
Verbal Noun 2 - a’ ceannach
- Tha mi a’ ceannach fèileag. - I am buying a kilt.
- Tha mi a’ ceannach IRN BRU. - I am buying IRN BRU.
- Tha mi a’ ceannach taigeis. - I am buying Haggis."
Basically, I'm not sure what the rule is, but just try to remember that "a’ ceannach" = "buying", and "ag iarraidh" = "wanting".
Hope that helps though :-)
Uh...I answered this question as "she is buying shoes" and duo said I had a typo and the correct form is "she is buying shoe." Besides that English being wonky, I'm confused about whether this means that bròg/shoe specifically had to be singular here, since hovering over the dictionary showed both "shoe" and "shoes."
I understand your confusion with it sounding wonky when directly translated. However, the course throws in unusual sentences because it helps to learn the words themselves, rather than just learning them in a specific context. Some sentences come out very funny XD
As far as I can explain it as a learner, "bròg" means "shoe", and "brògan" is "shoes". The "a" before an object is assumed to be automatic where it would make sense to use it, unless tied to another word such as "the" or a possessive pronoun. So in this sentence, "bròg" means "a shoe". It's just one of those little quirks that you just get used to in time :)