Complex sentence: On his return he was appointed assistant to Reverend James Fitch, of the North church of Portsmouth, was ordained pastor in 1747, and continued in that charge till 1774, when he became president of Harvard.
Split into simpler sentences: On his return, he was appointed assistant to Reverend James Fitch of the North Church of Portsmouth. He was ordained as pastor in 1747, and continued in that charge till 1774, when he became president of Harvard.