Beyond niet (not) and geen (no), Dutch negation uses a set of negative pronouns and adverbs: nooit (never), niemand (nobody), niets (nothing), nergens (nowhere), nauwelijks (hardly or scarcely), niet meer (no longer), nog niet (not yet), helemaal niet (not at all), nog nooit (never yet). Each of these combines negation with a specific meaning that would require multiple words in English.
Dutch generally uses single negation, not double negation: Ik heb niemand gezien, not Ik heb niemand niet gezien. Adding niet after niemand is non-standard. However, sentences with negative polarity items are common: iets becomes niets in negative contexts, iemand becomes niemand, ergens becomes nergens. This automatic replacement is a key feature of Dutch negation that learners need to internalise.
Partial negation is possible through word stress and the use of niet with just part of the sentence: Niet iedereen weet dat (Not everyone knows that), Niet altijd (Not always). The negative prefix on- also creates negated adjectives: onmogelijk (impossible), ongelukkig (unhappy), onvoldoende (insufficient). Learning these forms expands your ability to express precisely what is not the case and makes your Dutch sound much more precise and natural.