*ne (unstressed *ni)
This word was a general negation particle, and always preceded the word that is negated. Since imperative forms of verbs were always placed first in the sentence, the particle could not precede it. Therefore if the imperative form of a verb was to be negated, the subjunctive was used instead. If a verb with an adverbial prefix needed to be negated, the word was normally inserted between the prefix and the main verb stem.
This word does not survive in its original use in any modern language. Over time it usually became strengthened by other words, creating phrases such as 'never', 'not at all' or 'not a thing'. Eventually these phrases became the normal way of negating phrases, and this word fell into disuse.