Complex sentence: By age 25, Carné had already directed his first film, one that marked the beginning of a successful collaboration with surrealist poet and screenwriter Jacques Prévert that lasted for more than a dozen years during which they created films that defined French cinema of the day.
Split into simpler sentences: By age 25, Carné had already directed his first film, one that marked the beginning of a successful collaboration with surrealist poet and screenwriter Jacques Prévert. This collaborative relationship lasted for more than a dozen years, during which they created films that defined French cinema of the day.