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Regular "не" always works as "minus with minus makes plus".
For example: "Не задавай ненужных вопросов" - "Do not ask unrelated questions" means "Ask related questions" after all.
But pay attention...

P.S.

So, let's parse complex case: "Я никогда ничего не буду недооценивать".
Well, we have 4 no-particles here. "не буду" (will not) tells us we have negation, so we convert ни-words to...

First of all there is special particle "ни-" in russian. If it's used as a distinct word - it means "neither". But if it's joined as prefix to word it becomes "no-".
-Где это было? - Нигде. (Where...