does 'perthnasau' break down into anything? (like weekend 'penwythnos = end eight night)
Not really. "Perthyn" means "To belong/To relate to" so I imagine that's how it's developed.
The Discuss button is not working in some of the exercises in this section.
Please report that kind of technical issue in the Duo troubleshooting discussions. The course teams have no control over the internal workings of Duolingo, and the Duo technical staff do monitor the troubleshooting discussions.
It is always useful for the technical problems to be reported so that they can be noticed, logged, and (hopefully) fixed. You can find the discussions if you look on the web for 'duo troubleshooting discussion' or similar.