Complex sentence: He finished his primary education, but in 1917, when he was 15 years old, he felt that in Manati he didn't have a future and left his home; he then moved to San Juan, the capital city of Puerto Rico.
Split into simpler sentences: He finished his primary education but, in 1917, when he was 15 years old, he felt that in Manati he didn't have a future and left his home. He then moved to San Juan, the capital city of Puerto Rico.