Complex sentence: The President who is unable to exercise the powers and duties of the office remains the President of the United States during the period when there is an acting president; in other words, the incapacitated President does not become an ordinary citizen.
Split into simpler sentences: The President who is unable to exercise the powers and duties of the office remains the President of the United States during the period when there is an Acting President. In other words, the incapacitated President does not become an ordinary citizen.