Edit: problem solved. What went wrong, is that the lines I thought were single, were in fact closed donut shapes. Breaking them up at the ends did the trick.
the problem I have is quite simple. This is what happened:
1. I used the pie tool to make circle segments, just single line segments
2. I converted the segments to curves (so basically, I had two simple lines shaped like a c that I tried to change into an s)
3. I tried to join the curves in several ways; I tried the "join curves" command in the correct way (checked it several times), tried the "add" command, tried the "close curve" thing, but nothing seems to work.
So, my problem is that the circle segments created with the pie tool do not obey to some node actions. The actions that don't seem to work, are "join curves", "break curve", and "close curve". On top of that, the "add", "subtract", "xor", "divide" and "intersect" tools don't do anything; the divide tool even makes the curves disappear altogether. All these things work fine on other curves, and also on circles made with the circle tool. I hope this is enough information for you.
Thank you for your time,