Complex sentence: He completed 3 years of high school at Sabana Grande High School before his family moved to San Juan; here he attended the "Escuela Superior Central" (Central High School) of Santurce.
Split into simpler sentences: He completed 3 years of high school at Sabana Grande High School before his family moved to San Juan. There he attended the "Escuela Superior Central" (Central High School) of Santurce.