Negation: δεν and μη(ν)
Greek uses TWO negators, chosen by mood. δεν negates a plain statement (the indicative): Δεν γράφω (“I don’t write”). μη(ν) negates a command or a να-clause (the subjunctive): Μην γράφεις! (“Don’t write!”), σου λέω να μην έρθεις (“I tell you not to come”). Greek also uses DOUBLE negation (negative concord): the verb keeps δεν/μην AND takes a negative word - Δεν ξέρω τίποτα (“I know nothing”), Δεν ήρθε κανένας (“nobody came”). The final -ν of δε(ν) and μη(ν) follows the same nu-movable rule as στον/στη(ν).
The rule
Greek has two negators and the mood chooses between them. δεν negates a plain statement (the indicative): Δεν γράφω. μη(ν) negates a command or a να-clause (the subjunctive): Μην γράφεις!, Θέλω να μη μιλάς. Get that pair right and you have the whole system, because there is no “do” auxiliary to arrange and no word order to change - the negator simply sits in front of the verb, after any clitic pronoun. Greek also uses double negation (negative concord), which English forbids: the verb keeps δεν or μην and takes a negative word - Δεν ξέρω τίποτα, Δεν ήρθε κανένας. Both negators end in a movable -ν, so before most consonants they appear as δε and μη.