Complex sentence: A-not-B error is, in the theory of cognitive development of Jean Piaget, a particular error made by young children during substage 4 of their sensorimotor stage.
Split into simpler sentences: A-not-B error is a phenomenon uncovered by the work of Jean Piaget in his theory of cognitive development of children. The A-not-B error is a particular error made by young children during substage 4 of their sensorimotor stage.