"I am not playing the violin inside but outside."
Translation:Nem bent hegedülök, hanem kint.
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.
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.
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.