In bwydo'r ci, the contraction is bwydo yr ci (feed the dog).
The definite article yr "the" takes one of these three shapes:
- 'r if it follows a vowel (as here: bwydo yr becomes bwydo'r)
- otherwise y if it stands before a consonant (as in y ci "the dog" when there's no word ending in a vowel in front)
- otherwise yr (as in yr afal "the apple")