The adverb 曾经 "once"
曾经 is used to indicate an indefinite time in the past.
The adverb 曾经 
is used when referring to something that happened a long time ago.
In a sentence, it is placed before a predicate, which can be expressed by either a verb or an adjective.
is used when referring to something that happened a long time ago.
In a sentence, it is placed before a predicate, which can be expressed by either a verb or an adjective.Scheme
subject
曾经
predicate
Pay attention
In sentences with 曾经, the aspectual particle 过 often follows the verb.
often follows the verb.Examples
听说你曾经来过巴黎
I heard you were once in Paris
tīngshuō nǐ céngjīng láiguò bālí
人们曾经认为地球站在三头大象上
People used to think that the Earth was placed on three elephants
rénmen céngjīng rènwéi dìqiú zhàn zài sān tóu dà xiàng shàng
市政府曾经在我们家旁边
The city hall used to be next to our house
shì zhèngfǔ céngjīng zài wǒmen jiā pángbiān
曾经 cannot be used in negative sentences.
However, you can use both 不曾 and 未曾 "never" before a predicate in written language.
and 未曾 "never" before a predicate in written language.Examples
他们一直写信 ，不曾见过面
They’ve never met, and have only wrote letters to each other
tāmen yìzhí xiě xìn, bùcéng jiànguò miàn
他从小就没有父母 ，未曾感受过家庭的温暖
He hasn’t had parents since childhood and has never felt the warmth of family
tā cóng xiǎo jiù méi yǒu fùmǔ, wèicéng gǎnshòuguò jiātíng de wēnnuǎn