Complex sentence: Around 1818, when he was about 28 years old, Menominee began to preach among the Potawatomi, after he claimed to have been called to the task by the Great Spirit.
Split into simpler sentences: Around 1818, when he was about 28 years old, Menominee began to preach among the Potawatomi. He claimed to have been called to the task by the Great Spirit.