Correct

曾 vs 曾经

/ céng vs céngjīng /
Strokes

Formula

1
不曾 (never before)
2
未曾 (never before)
3
何曾 (used in a rhetorical statement to mean "have not", "did not")
4
似曾 (seem to be)
5
时曾相识 (seen to have met before)

Definition

曾 and 曾经 both are Adverbs, which means [once before].

Example

曾 and 曾经 both words indicate that sth. once happened before. 曾 is more literary than 曾经.
1
céngjīng
曾经
  
(
zēng
  
)
guò
hǎinándǎo
海南岛
  

He has been to Hainan Island once.

2
Zhè
这里
céngjīng
曾经
  
(
zēng
  
)
shì
dài
古代
de
zhànchǎng
战场
  

The place was once an ancient battlefield.

曾 can be used in 不曾 (never before), 未曾 (never before), 何曾 (used in a rhetorical statement to mean "have not", "did not"), 似曾 (seem to be), etc. 曾经 cannot be used in these phrases.
不曾 (never before) 未曾 (never before) 何曾 (used in a rhetorical statement to mean "have not", "did not") 似曾 (seem to be)
3
cónglái
从来
zēng
不曾
jiàn
guò
xuěshān
雪山
  

I have never seen a snowy mountain.

4
wèicéng
未曾
xiǎng
guò
zhè
pān
dàoli
道理
  

He has never thought of this argument.

5
shuōèr
说一不二
  
,
céng
何曾
shuōhuà
说话
suànshù
算数
guò
  
?

I stand by my words. Have you ever known that I do not?

The negative form of 曾经 is not 不曾经, but 不曾 or 未曾.
6
céng
不曾
guò
xiānggǎng
香港
  

He has never been to Hong Kong.

7
de
huà
zài
shēngqián
生前
wèicéng
未曾
biǎo
发表
guò
  

His paintings were never published during his lifetime.

曾 can be used in idiomatic phrases such as 时曾相识 (seen to have met before)
时曾相识 (seen to have met before)
8
duì
zhè
这里
de
jǐng
景物
hǎoxiàng
好像
zēngxiāngshí
似曾相识
  
,
dàn
yòu
de
记得
shí
何时
céngjīng
曾经
lái
guò
  

He seemed to be familiar with the scenery here, but he couldn't remember when he had been here.

9
men
他们
chū
初次
xiāngjiàn
相见
dōu
yǒushí
有时
zēng
xiāngshí
相识
de
gǎnjué
感觉
  

The first time they met, they all had the feeling of having known each other.