"Penguins cannot fly."
Translation:Pinguine können nicht fliegen.
Pinguine können nicht fliegen -> the cannot fly. Because they just can't^^.
Enten fliegen nicht. They could if they wanted to but they don't. (Pinuine können es nicht, Enten schon.)
können - > to be able to
to do (in this case "fliegen") -> an ability, you can use it or not.
The normal thing to say in German is that they don't fly. The normal thing to say in English is that they can't. Neither is exactly wrong in the other language, but both are needlessly awkward. "Pinguine können nicht fliegen" sounds like they're supposed to be able to fly, but have somehow been disabled from doing so. And "Penguins don't fly" sounds like they can, but just choose not to.