Hmm… doesn’t Sc. Gaelic use the genitive in this construction only with the definite article (so ag ithe feur eating grass vs ag ithe an fheòir eating the grass)? Would anyone these days use the genitive feòir in such a sentence, even in very formal or high style? (Honest question, I have no idea, just repeating what I read on taic/akerbeltz/etc.)
I see Colin Mark gives an example Tha i a’ glanadh uinneig(e). (she is cleaning a window) (Gaelic Verbs: Systemised and Simplified, §A.3., p. 140), but then notes that it is becoming common to drop genitive in indefinite noun and gives examples: tha i a’ glanadh uinneog contrasting to tha i a’ glanadh na h-uinneig she is cleaning the window.
If it is still used when using higher style (eg. scientific articles, narration in prose) then perhaps it should be accepted, but if it went out of use, then even if considered old-fashion but correct – not necessarily. English course for foreigners, for example, IMO should accept whom were you talking to? alongside who were you…?, but shouldn’t necessarily accept very old-fashion where art thou? in place of where are you? (even though I think this might still be used in some Northern English dialects).
I think things may have changed very rapidly since I learnt Gaelic not that long ago. They were definitely teaching that you should - in principle use the genitive even with an indefinite noun but that it was no longer usual. Our text books included ones by Mark, cited by silmeth above. But I was taught to always use it with a definite noun. Has this changed as you say 'you might get off with using the genitive with definite nouns'?
I tried looking through Mark's dictionary and my book of Litreachan do Luchd-ionnsachaidh and basically I got bored. Examples of verbal nouns followed by an indefinite direct object that is not itself part of a possessive structure are vanishingly rare. So if we ignore
Tha a' chaora ag ithe an fheòir
Tha a' chaora ag ithe feur na talmhain (phrase from Rev. ch. 8)
Tha a' chaora ag iarraidh feur ithe
then I simply could not find any examples in 10 minutes.
I think it is so uncommon in real life (as opposed to artificial examples) that it is almost academic. Perhaps the rarity of examples where you might use it means that the forms shown in examples 2 and 3 above are taking over? I think I was told that the non-use of the genitive was spreading from the suspended genitive (example 2), which was itself an innovation a few centuries ago.
There are two issues here.
The feminine singular definite article causes lenition (in all Celtic languages). In Gaelic and Irish c changes to ch.
Uniquely in Gaelic, the n is dropped when the h is inserted. I don't know why. It came in a few centuries ago with c and g the last to undergo the change, so you might actually see an chaora as late as the 20th century.
Is that a longer answer then you wanted?