曾经 (céng jīng) means once, at one time, or used to, indicating that something was true or happened in the past but may no longer be the case. It appears before verbs (曾经去过, once went) and often carries a nostalgic or reflective tone. It pairs naturally with 过 (guò, the experiential aspect marker) and 了 (le). Compared to 以前 (yǐ qián, before), 曾经 tends to be slightly more literary and emphasises the contrast with the present.