"May I have an orange please? Yes."

Translation: Ga i oren plîs? Cei.

[VJRees]

I wrote Ga i oren os gwelwch yn dda? Cei and it said there was a typo in my answer, Gaf instead of Ga, is that correct and if so why Gaf?

[ibisc]

What you have written there is fine, and it is one of the alternative answers that should be accepted by Duo. Gaf i is just a common variation of Ga i, and the course accepts both forms.

[SueBeaudry]

I didn't ask before but I'm curious: Is "Cei" a version of "Okay?" or something really Welsh?

[mizinamo]

It's really Welsh -- a form of the verb cael (get, receive, be allowed to).

If several people were asking for oranges, it would have been Gawn ni orennau, plîs? Cewch. -- with a different form since now the answer is "you [plural] may" rather than "you [singular] may."

[ibisc]

See the course notes (https://forum.duolingo.com/topic/924/hot and look for 'Course tips and notes)

We suggest reading the course notes for each new section as you start it. For example - https://www.duolingo.com/skill/cy/Gai/tips-and-notes