曾经 + Verb + 过 (once / formerly)

"Once, back then"

• once曾经céng jīng = "once / formerly" — something true at some point in the past.
• I / me我wǒonce曾经céng jīng去过qù guòthere那儿nà ér = "I've been there before."
• It usually pairs with the experience marker (experienced)过guo.

Subj + once曾经céng jīng + Verb + (experienced)过guo

• once曾经céng jīng sets a past experience; (experienced)过guo marks it as "done at some time".

he他tāonce曾经céng jīngbe当dāng(experience)过guòteacher老师lǎo shī。

• once曾经céng jīngat or in the very same...当dāng(experienced)过guoteacher老师lǎo shī = "(he) was once a teacher."

Recap

Read the full reference: HSK 5 grammar — Adverbs.