절대(jeol-dae, jeoldae)
Expresses strong negation, very common in everyday conversation
When to use
everyday negation emphasis, strong prohibition, negation in emotional expression
The nuance
절대 is used in negative sentences to strongly emphasize that something will never happen or be done; it is natural in spoken Korean.
결코(gyeol-ko, gyeolko)
Strong negation in negative sentences, mainly used in written or formal contexts
When to use
literary negation emphasis, formal speeches or writing, legal or official documents
The nuance
결코 also expresses strong negation but is mainly used in literary or formal contexts; it sounds unnatural in casual spoken Korean.