"Ga i losin?"
Translation:May I have some sweets?
'Wedi' is used before a verb to express the perfect tense in English. To show a completed action.
I have gone = Dw i wedi mynd
I have come = Dw i wedi dod
'Ga i' is used to express possessing something, the use of 'have' in English to show possession.
Ga i afal? = May I have an apple?
Ga i siocled? = May I have chocolate?
This is very well explained in the notes for this section, available on the web based Duolingo course.
I know that "may" is said to be correct and "can" is colloquial. But as 90% of English speakers always say "can": at what time does this become correct in a living language?