Complex sentence: He was only there for a year before he was moved to a parish school to be taught by Alexander Paterson, under whom he made such progress that he graduated from school at the age of fourteen.
Split into simpler sentences: He was only there for the span of a year until he was moved to continue his education at a parish school taught by the man Alexander Paterson. It was by Alexander Paterson's instruction that John made unbelievable progress until he graduated from school at the age of fourteen.