In the unit where they are encountered. When using the sentence 'May I have a drink' 'diod' changes to 'ddiod'
'Ga i ddiod?' = May I have a drink?
This is the process of initial consonant softening which occurs in all of the Celtic languages. There is further explanation in the notes for this unit.
Actually consonant softening is common to many languages eg English 'Knife' and 'Knives'.