Comparing "cengjing" and "yijing"

曾经 and 已经 both are adverbs that are used to indicate something happened in the past and sometimes can be used interchangeably. 曾经 is used in formal situations, whereas 已经 is used in informal and formal situations. The biggest difference between these two is 曾 is closer to “once” in English, expressing an action that took place in the past and has already finished up. On the other hand, 已经 is closer to “already”, expressing an action that took place in the past and has not necessarily finished up. A final difference, is that only 曾经 is used to express something that took place in the distance past (think “once upon a time, long ago…”).

	曾经 (céngjīng)	已经 (yǐjīng)
Meaning	already	once
Part of Speech	adverb	adverb
Formality	formal	flexible