Complex sentence: He was born in the Dolpo region of modern Nepal but in 1309, when he was seventeen, he ran away from home to seek Buddhist teachings, first in Mustang, and then in Tibet.
Split into simpler sentences: He was born in the Dolpo region of modern Nepal. However, in 1309, when he was seventeen, he ran away from home to seek Buddhist teachings, first in Mustang, and then in Tibet.