I find it irritating that we are just presumed to know these contractions.
I agree. No mention of this contraction before being marked wrong for not knowing it. Irritating indeed.
The course notes for the sections on 'Numbers' explain this. We recommend reading the notes for each new section as you start it.
Go to https://forum.duolingo.com/topic/924/hot and see 'Course tips and notes'.
But there are no course notes on the app, which most people use.
If you have an internet connection, the notes are available by using a web browser.
If you go to the 'duome' link mentioned in the 'Course tips and notes' discussion, you can take a copy for off-line reference. Just remember to update it from time to time.
Why is it not cwn?
Welsh uses a singular noun when it follows directly after a number.
The course would be greatly improved if more exercises had audio. No sound when working on new vocabulary hinders learning.
We have sometimes had to disable audio on particular sentences or words if Duo's text-to-speech is too garbled.
'Five dogs' translates to 'pump cwn', not 'pum ci'.
There is no word before 'pump ' to cause a change.
The plural of 'ci' is 'cwn'.