Complex sentence: Sister Margaret seems to be the only one who sympathizes with the children; she tries to help Billy open up and play with the other children, but Billy is constantly subject to Mother Superior's scrutinizing eyes and regularly punished.
Split into simpler sentences: Sister Margaret seems to be the only one who sympathizes with the children. She tries to help Billy open up and play with the other children, but Billy is constantly subject to Mother Superior's scrutinizing eyes and regularly punished.