"once he qualified as a teacher."在句中是什么成分,修饰谁?

问题描述:

"once he qualified as a teacher."在句中是什么成分,修饰谁?
原句是“Gnang studied in the United States but returned to the village once he qualified as a teacher.”

是个时间状语从句,条件从句
这句话的时态有点不对,
翻译:Gnang一直在美国学习,后来一拿到教师资格证就回镇子教书了