Isst or frisst?
Are those words interchangeable? If not, why are they used differently?
This question is answered in the tips and notes for the "Animals 1" unit: https://www.duolingo.com/skill/de/Animals-1
I recommend using the website to learn, not the mobile apps, because the mobile apps don't show you the tips and notes yet (nor even tell you that they exist).
Lots of things are explained in the tips and notes.