I wrote "Hun forstår ikke det jeg sier" and it was accepted. I think of "det" as "that which."
what is the difference between skjonner and forstar?
In the alternate answer, is ei another word for ikke? "Hun forstår ei hva jeg sier." And what's wrong with "forstår ikke"?
Yes, "ei" is a less common variant of "ikke".
Unless you speak one of the few dialects that favour it there is no reason to use it outside of a few set phrases, but it's good to be able to recognise it.
There's nothing wrong with "forstår ikke". :)
Are føstar and skjønner interchangeable?