ne

Ne sits directly in front of whatever it denies, which is usually the verb, and on its own it is the word for "no". There is no do-support and no second particle to remember: mi ne venas is the whole of "I am not coming". Keep one negative per clause, because the neni- words already contain it, so neniu venis means "nobody came" with no extra ne. For the opposite of a word rather than the denial of a sentence, use the prefix mal-: bona gives malbona.