Zhǐyǒu dāngshìrén cái liǎojiě shìqing de zhēnxiàng.
Only the person involved knows the truth of the matter.
Tips
usage
当事人 is used in both legal and everyday contexts. In law, it refers to the parties in a case (plaintiff/defendant). In daily use, it means the person directly involved in an event or situation.
