曾 (céng) is a formal adverb meaning 'once, ever' — it emphasizes that something happened in the past, often paired with 过. Compared to the more common 曾经, 曾 is slightly more literary and concise; both express past experience but 曾 is rarer in everyday speech. Unlike 已经 ('already'), which focuses on a change of state, 曾 simply marks that an event occurred at some point.