Complex sentence: He started serving in the Han imperial palace as an attendant holding the rank of "jishi shengzhong" (給事省中) before he was promoted to "xiao huangmen" (小黃門) during the reign of Emperor Huan (146--168).
Split into simpler sentences: He started serving in the Han imperial palace as an attendant holding the rank of "jishi shengzhong" (給事省中). He was promoted to "xiao huangmen" (小黃門) during the reign of Emperor Huan (146--168).