Complex sentence: In 1973 he was confined in a reformatory after stealing a motorbike and was released in 1975, when he committed his first serious crime when he raped a girl with his friends.
Split into simpler sentences: In 1973 he was confined to a reformatory after stealing a motorbike, and was released in 1975. About that time, he committed his first serious crime when he raped a girl with his friends.