"Does he understand what I am saying?"
Translation:Forstår han hva jeg sier?
FWIW, det was accepted for me. I don't know whether hva sounds better or not. I think I'm getting a pretty good handle on figuring out when to use hva and when to use det, and I think this may be one of those sentences where either hva or det is acceptable ... but I still don't know if they're equally acceptable, or if one sounds slightly better than the other. Such sentences make figuring out exactly what the rules are all that much more confusing for us Norsk learners. :-)