"Who is the mother?"
Translation:Quis est mater?
It should also be accepted now, but unfortunately it takes some time for the changes we make in the Incubator to be active for users (sometimes as long as two weeks).
Still, please report (with the button in the lesson, not in the discussion) if it's not accepted, it's still possible something got missed!
Apparently it can go either way.
There is not a database of acceptable typos the way there is a database of acceptable answers. The correction algorithm, which is programmed by the site devs, is a delicate balance of determining the difference between a typo and an error. It generally allows one wrong letter per word to slide by as a typo. It is supposed to mark it wrong if that one-letter difference makes a real word in the language, but sometimes that doesn't happen. There is nothing the volunteer course contributors can do about that.