WHAT'S WRONG WITH הן לא יודעות את זות?
I wanted to know the same thing. Why does one correct answer use את before the article and the other does not?
Masculine form is the default.
Why can't "הם לא יודעים זה" be accepted?
Because you need את.
why do we need the word "את"?
What don't they know? ---> This. So "this/זה" is the direct object and with a direct object you use את