How do you write ‘filly’ as in lesson 3 Course 2, where ‘filly’ (failed to) is used for couldn’t? e.g. I couldn’t phone.
Also…should I be creating a topic to ask this question or is there a better way to ask Course specific questions?
Ffili, probably. Possibly ffeili.
“Ffili”.
Like a lot of words, especially “dialect” words, you will see it spelled (and indeed hear it pronounced) a lot of different ways.
Probably “Ffili” along with “Ffaelu” are possibly the commonest ones you will see.
As to course specific questions, I believe that yes, now there are no longer any dedicated threads, so you should just create a topic to ask a question. 
As hectorgrey says (didn’t see his answer when I replied above), “ffeili” is another possibility.
“Ffili” really spells what you represent by “filly”, for what it is worth.
I think it will be more likely to be ffaelu but am only a learner like you
Thank you, I am happy with all the suggestions.I know the Google translate tool is not an authority, especially for dialect, but it was interesting to see what it threw up for the suggestions above…
ffaelu - bust
ffili - filly
ffeilu - ffeilu
This was Welsh to English.
For English to Welsh, I couldn’t find anything looking like ‘filly’ for ‘can’t’ or ‘failed’ etc.
Good old Google 
I should have looked a bit closer. Underneath the main returns there is sometimes an alternative. When I looked closer, I found for ffaelu
fail
methu, fethu, ballu, diffygio, ffaelu, mhallu