Which one is the correct way to say 'do not forget': 'n'oublie pas' or 'ne pas oublier' ?

Hi Shabbir,

The difference is who it is adressed to -you use ne pas + infinitive when it is aimed at everyone, as a general request/caution. e.g.

Ne pas se pencher par la fenêtre (Don't lean out of the window)

Ne pas fumer (No smoking)

If you say "n'oublie pas d'emporter ton passport !" you are reminding the person you are talking to only. In contrast to the following general caution-

Ne pas oublier son passeport est important si on va l'étranger.

Hope this helps!