Verbs take different forms depending on the subject, a bit like how it's "I am" and "you are" and "he is"... it's not always "is" or always "am".
Are you using a mobile app to learn? Those do not provide access to the tips and notes that explain German grammar. I would recommend that you use the Duolingo website instead.
Have a look at the tips and notes for the "Animals" skill: https://www.duolingo.com/skill/de/Animals-1
That shows the conjugation of the verb essen "to eat".
I keep putting the Women eats. And I'm English. I know perfectly well that it means one woman, and I am English. Whoops
Die Frau comes from Fraulein which the Germans stopped using around 2001-2002, in German class they mentioned the Die Frau is for general females who we are talking about. The Woman, or meine Frau means girlfriend or wife and there are a few other words used for that title but Germans have a tougher time translating our words because they do have multiple meanings! I'd say "Die Frau isst" by itself should mean "the woman is eating" because everyone eats and unless you specify a particular food, we would thing that sentence is continuous!
There is a good explanation of accusative case here https://www.thegermanproject.com/german-lessons/nominative-accusative
Basically, the accusative case can change the object of the sentence.
ORLY? Did you not learn about verbs and how they change?