"The butterfly enters through the open window."
Translation:La farfalla entra per la finestra aperta.
tomerjar: I believe that "nella", or any contraction meaning the english "in the", actually refers to a noun being physically inside another noun. The butterfly in this case is not physically inside the window itself as an object, but merely passing through it. I may be wrong about this, however, I feel very positive that this is the italian meaning.