I can't help but notice that the Welsh word 'dafarn' is very similar in sound to the English word 'tavern', which is a synonym for a pub or bar. Does anyone know if these are related or just a coincidence?
They are both ultimately from Latin taberna; English got it via French according to EtymOnline, Welsh directly from Latin according to GPC (along with lots of other borrowings such as gwyrdd or ffenestr).
The base word in Welsh is tafarn (x). It is a feminine noun, so the t- changes to d- after y (the) and un (one):
- y dafarn, un dafarn
(x - Except in parts of mid-Glamorgan, apparently, where it is often a masculine noun. Stick to learning it as a feminie noun, though.)
You've commented on one of the nouns that can be both genders, so to answer your latter question "Y dafarn" and "Y tafarn" are both equally correct. This isn't true for all nouns i.e "The temple" is "Y deml" because "Teml" is a feminine noun, and therefore "Y teml" would be wrong.
Does "t" always change to "d" after "y"?
No, only if "y" is the definite article and the word following it is a feminine noun such as tafarn.
A masculine noun such as tad "father" would not change: y tad "the father".
As mizinamo says, this only happens to feminine nouns after the definite article. This is called the soft mutation, and it's not just "t" which changes:
- b -> f
- c -> g
- d -> dd
- g -> -
- ll -> l
- m -> f
- p -> b
- rh -> r
- t -> d
(As mizinamo and ibisc pointed out, ll and rh don't mutate after the definite article, alongside a couple of other words.)
y (=definite article), un, yn (before noun/adjective), mor/cyn (=as) all project 'weak' soft mutation, which means that they do not mutate ll, rh.
As we explain in the initial course notes, the course does not include slang terms, dialect terms (unless very common) or formal language patterns on this course. It is designed for beginners following the Dysgu Cymraeg courses for adults sponsored by the Welsh Government.