Complex sentence: John Quincy Adams, former President of the United States and at that time a U.S. Representative from Massachusetts, had agreed to argue for the Africans, but when it was time for him to argue, felt ill-prepared.
Split into simpler sentences: John Quincy Adams, former President of the United States and at that time a U.S. Representative from Massachusetts, had agreed to argue for the Africans. When it was time for him to argue, he said he felt ill-prepared.