曾 (céng) is a literary adverb meaning once or formerly, indicating a past experience. In classical and formal Mandarin 曾 alone marks past experience, parallel to the 过 (guò) experiential aspect marker in modern speech. In everyday Mandarin it typically appears in the compound 曾经 (céng jīng, once upon a time / used to), which is more colloquially natural. The negative 未曾 (wèi céng) means have never, appearing mostly in formal or literary contexts.