Complex sentence: The Methodist Episcopal Church (MEC) was the oldest and largest Methodist denomination in the United States from 1784 until 1939, after which time it merged with other Methodist churches to form what eventually became the present-day United Methodist Church.
Split into simpler sentences: The Methodist Episcopal Church (MEC) was the oldest and largest Methodist denomination in the United States from its founding in 1784 until 1939. In 1939, The MEC merged with other Methodist churches to form what eventually became the present-day United Methodist Church.