That was a very helpful answer, Manar. Note that you could have answered directly (by the pink button) rather than limit it to the comment section. I've just realized that the only reason why the Shami and MSA variants differ from each other is because the Shami indicative imperfect verb requires a "b-" prefix and because, when acting as a pronoun rather than a participle, "maa" means "not" rather than "what." Doesn't look so different and complicated now.