Indeed, "_ne_" can always be omitted in _spoken_ French ("_ne ... que_", "_ne ... jamais_", ...).

If you are trying to speak or write in a formal way however, I would recommend _never_ to omit it in an official document<sup>1</sup>.

When spoken, it is also usual to shorten it, just pronouncing the _N_ (a sort of a contraction). When speaking formally, "_Je n'suis pas d'accord._" will be better than "_Je suis pas d'accord._"

Additional note: in literary French, "_pas_" can be omitted in "_ne ... pas_". This is very rarely heard in spoken French however, and is specific to "_ne ... pas_".

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<sub>1. My first instinct would be to compare it to the inflected negative auxiliary verbs (e.g. the "don't" contracted form) in English, which I would avoid in formal documents. Unlike the French omission though, the contraction is not considered unformal according to [American recommendations](http://www.plainlanguage.gov/howto/guidelines/FederalPLGuidelines/writeContract.cfm) (which can be summed up as "write as you talk") and [English ones](http://french.stackexchange.com/questions/14020/omission-of-ne-in-ne-personne/14022?noredirect=1#comment26537_14022) (thanks to [mikeyreilly](http://french.stackexchange.com/questions/14020/omission-of-ne-in-ne-personne/14022?noredirect=1#comment26495_14022) for pointing that out).</sub>