Complex sentence: Jack Gantos was born in Mount Pleasant, Pennsylvania and his family moved around the United States until Jack was a junior in high school, when his family moved from their home in Fort Lauderdale, Florida to San Juan, Puerto Rico.
Split into simpler sentences: Jack Gantos was born in Mount Pleasant, Pennsylvania and his family moved around the United States until he was a junior in high school. When Jack was a junior in high school, his family moved from their home in Fort Lauderdale, Florida to San Juan, Puerto Rico.