"He cooks soup."
Translation:Er kocht Suppe.
It takes a '-t' ending because it's in the present tense and it's in the third person singular- that is, the verb is being done by one he, one she or an it.
I recommend reading the tips and notes before every lesson, as they explain a lot of this stuff. They're only available on the web version so it's a good idea to switch to it if you're using the app.
In the meantime, here's some stuff about verb endings: