사고(sa-go, sago)
An unexpected physical accident or a negative outcome
When to use
Traffic accident, Industrial accident, Physical damage, Everyday negative incidents
The nuance
Use 사고 mainly for physical accidents like traffic accidents or industrial disasters. It is rarely used in emotional or social contexts.
사건(sa-geon, sageon)
Used for socially, legally, or historically significant incidents
When to use
Criminal case, Social issue, Legal matter, Historically important incident
The nuance
사건 is used for crimes, social issues, historical incidents, and other official or significant matters, distinct from physical accidents.