"I am not playing the violin inside but outside."

Translation: Nem bent hegedülök, hanem kint.

[carolgoslen]

It is wrong to use én to begin the sentence? I cannot decide when the pronoun is necessary or is not necessary when the sentence is not in 3rd person singular.

[jsiehler]

It's fine to use it here. Nearly always, it's fine to include it or omit it. Duo just isn't entirely consistent about accepting all the possibilities yet.

[vvsey]

In any person, not just the 3rd singular: the pronoun is used for emphasis only. If you don't want to emphasize it, you omit it. The information is already contained in the verb at all times.