I never would have thought this sentence would be useful but yesterday I met a little girl wearing beach shoes with a tortoise and a crab on them and I was able to chat to her
Yes, the definite article changes to 'r after a vowel.
Eg 'The dog' = Y ci
'The pig and the dog' = Y mochyn a'r ci
The definite article also changes to 'yr' before a vowel.
e.g 'The sweets' = Y losin
The apple = Yr afal