This is the 70th most frequent Chinese word.
曾
"曾" means "once" or "previously."
In this sentence, '曾' (céng) indicates that the event happened in the past and has a connection to the present.
我曾去过北京。
I have been to Beijing before.
Here, '曾' (céng) suggests that the subject had this status at some point in the past.
她曾是这所学校的学生。
She was once a student of this school.
'曾' (céng) in this context emphasizes a past occurrence that is being recalled.