How do FErs describe not being able to see across a plain forever? it just seems logical to me that you can't see forever because of the curve of the earth.
also, do they beleive that the earth is the only flat planet in the universe? or that they're all flat?
I believe they think the earth is the only flat planet. Not sure about the scope of this argument (in the solar system, in the galaxy, in the universe, etc).
Also, the flat earth as an infinite plane has been discussed at length (though I still don't see how it is feasible). But it is one of multiple earth theories out there. Also, there is apparently bendy light which makes it look like the earth is round.