Translation:Is the sofa below you hard or soft?
Realistically, we'd just say "the sofa [that] you're sitting on". "the sofa under you" could conceivably be correct, although too poetic for everyday use. "the sofa below you" is definitely wrong - it suggests, say, the sofa downstairs; a sofa a floor or two directly below where you're standing, which is weird thing to say. See an explanation for the difference between "under" and "below" here.