To investigate the factors determining why some children succeed at certain intellectual tasks while others of equal or near equal IQ, age, and motivation are unable to master the same task, 145 female and 144 male eighth grade students were administered "An Inventory of Piaget's Developmental Tasks" (IPDT). Labeled as concrete, operational, transitional, or formal operational thinkers according to inventory scores, the students were then given one of two grammar tests. Findings revealed significant correlation between grammar test scores, Piagetian stages, and IQ scores--indicating first, that the Piagetian stage as revealed by the IPDT is useful in predicting grammar test scores, especially when considered with IQ; and, second, that the task of identifying simple subjects and predicates is too difficult for most of the eighth grade students, including 47% of the students at the formal operations level, and a total of 74% of all students who took the test. The findings support Piaget's principle that children's cognitive development is a limiting factor in what they can learn at any given time. Results suggest that the abstract quality of grammar rules makes them too difficult for eighth grade students. (MM)