After being asked to translate «tu più non torni» on a casual conversation and with a sensible lack of context, I was a bit puzzled and assumed that «[you] don't ever come back» could be a valid translation.
Later, in that conversation I was told that it meant you will return no more, at least in Rodolfo's aria from La Bohème, and most of the usage examples on the treccani.it dictionary seem to confirm it.
Was my translation invalid?
The sentence cannot mean “don't ever come back”, which would require the imperative. In Italian the negative imperative is expressed with non and the infinitive
non tornare (mai) più
The form torni is a present tense, either indicative or subjunctive (but the latter can probably be excluded from context). The positive imperative is torna and never torni.
If the context makes the subjunctive possible, then it could be something like
Farò in modo che tu non torni più
I'll do so you don't ever come back again. But it is not imperative.
In the sentence there is a displacement of the adverb: tu non torni più would be more idiomatic. However, being the text of an opera aria, the displacement is not so surprising.
