When את is nessesary and when isn't?
It is only necessary to indicate a definite direct object. Anything that uses "the" or is a proper name or is otherwise definite.
Why is היא אוכלת את הבשר בקר wrong?
If you are using the whole "בשר בקר" you need the particle ה on the second noun because it is two-noun construction: בשר הבקר = beef
I wrote "היא אוכלת את בשר הבקר" on a translation exercise, and it was marked wrong. This looks like it should be reported.
I smell like beeeeef
ראשונה ככה כותבים
She is eating the beef. Translation:היא אוכלת את הבקר. this sounds horrible in Hebrew as בקר is cattle
בקר is both cattle and beef.