"Ar ôl i chi gael diod."
Translation:After you have a drink.
This pattern has no tense - you need to infer the tense from the surrounding context if there is one.
ar ôl i chi wedi diod would be incorrect - the ar ôl and the wedi duplicate the 'after' meaning, and it is missing the verb-noun cael, so the construction does not work at all.
- ar ôl i chi gael diod, byddwn ni'n mynd adref - 'after you have a drink, we'll go home' or 'after you have had a drink, we'll go home'
- ar ôl i chi gael diod, aethon nhw adref - after you had had a drink, they went home