One unifying theme is that understanding and producing sentences means relating, by a set of rules, an underlying structure to a more ambiguous, more arbitrary structure which is essentially the sentence that we hear. In the 1965 version of his theory, the distinction was between deep and surface structures, which were related by transformation rules , and the infant learns language through hypothesis-testing . In the 1981 'government binding' theory, fewer transformations were included, and the emphasis was more on the constraints . In contrast to Chomsky and Vygotsky, Piaget argued that there are cognitive prerequisites for language, i.e. children can only acquire something as complex as language when they have mastered sensori-motor intellectual skills such as object permanence. Evidence from language acquisition in apes shows that mastering object permanence is not a particularly crucial variable in language acquisition, since apes still cannot acquire grammar or the ability to reference objects in a 'human' way.
Ape language does, however, show the importance of social factors in language acquisition.
The view espoused above of how children acquire language sees the process as beginning at about 12 months with the child's acquisition of single words, with a vocabulary explosion around 18 months, when children also begin to combine words into simple sentences. Rather than acquiring single words, expressive children acquire their language in 'chunks'. The motherese hypothesis This states that the short, simple sentences, with exaggerated intonation and stress, typically used to talk to young infants act as a syntactic teaching device. Because the narrower and more biased the range of data, the more hypotheses can describe it, but as you increase the range of data, you tell the child more about the constraints on what can be said. Since both language and symbolic play involve representational abilities, it's not surprising that development in these two areas is related.