At my first encounter of "cei", I thought for sure that is was a loaner word from the USA word, OK or "Kay". But no, it's the future tense of the verb-noun "cael", to have. As is "chei", "cewch", and "chwech", depending upon the person or persons, and whether positive or negative.
cei and cewch are forms of cael. If they are used as negatives the they take an aspirate mutation to chei and chewch:
- [Cei di/Cewch chi] gawl i ginio. - You will have soup for lunch.
- [Chei di/Chewch chi] ddim cawl i ginio. - You won't have soup for lunch.
chwech - six. Nothing to do with cael.
These patterns are all explained with examples in the course notes. To find the course notes generally, go to https://forum.duolingo.com/topic/924/hot and see 'Course tips and notes'. The 'duome' link there is useful for browsing all the notes in one place. We recommend reading the notes for each new section as you start it.
I found your answer here really useful, thank you.
May I suggest that the first sentence, explicitly stating that cei is a form of cael, could usefully be added to the course notes for this unit?
It currently says 'ga i' is a form of cael, and I know from previous units that you answer a question starting with a verb with that same verb, but since we haven't learned to conjugate 'cael' (esp in the future) yet, and 'ga' and 'cei' look rather different, it's really useful reinforcement to be told 'cei' is also a form of 'cael'.
I think cei is used only in sentences that use a form of the verb cael, such as "Ga i oren? Cei." There is no word in Welsh that literally means "Yes". When you want to answer a question affirmatively in Welsh, you repeat the verb that is used in the question.
Here's a good explanation of how to say "yes" in Welsh:
Look especially at sequence #4.