[Coda]

"Learned" isn't really an appropriate translation there. 覚えている is more like "is learning", although the nuance could also be translated as "learned" depending on context (since it could be interpreted as "is in the state of learning").

The -eru conjugation is the "potential" form, meaning the "can do X" form; it's just a normal -ru verb after that conjugation, so -nai works the normal way.