Oddly enough, the word नहीं is not used with commands. One generally uses न or मत.
Although this has no bearing on learning the language: There is some theory by linguists, I believe, that नहीं is a contraction of न and है, kind of like "doesn't". न is the more elemental form of "no."
Indeed that would make a lot of sense. In Pali, a language of ancient India related to Sanskrit, "na" was already used as a negative (I don't know if there was any other word fulfilling that purpose actually). I would be very glad if any person knowledgable in Sanskrit or Pali could tell us more about this :-)