Complex sentence: Humphrey was made a royal ward on his father's death, under the control of Henry IV's queen Joan of Navarre; his minority was to be a long one, lasting the next twenty years.
Split into simpler sentences: Humphrey was made a royal ward on his father's death, and was placed under the control of Henry IV's queen Joan of Navarre. His minority was to be a long one, lasting the next twenty years.