HSK C3/W3/V4
Correct

/ zēng /
Strokes

Definition

once/already/ever (in the past)/former/previously/(past tense marker used before verb or clause)

Example

As an Adverb

once; formerly; sometime ago

1
Xiǎnérjiàn
显而易见
  
,
zēng
fēngliútǎng
风流倜傥
guò
  

He had obviously been romantic once.

2
nián
qián
  
,
zēng
guò
li
那里
  

I went there once a few years ago.