In Part III we look at the developing grammatical system, from its earliest beginnings up to (roughly) the point at which the child can be said to have acquired ‘the basic tools of language’ (see Karmiloff–Smith in Part IV). Understanding what this might mean in detail is an issue that we shall postpone to the next Part. Our immediate task is to look at how the grammatical system gets started, and how it then begins to develop over time.

First, by ‘grammatical system’ we understand a highly differentiated entity comprising vocabulary, morphology (derivational and inflectional), syntax, and the communicative functions which are expressed by these aspects of language structure. So, in reality, we are dealing with rather different subsystems, having their own structural properties and calling ultimately for some developing executive control which is distinct from them but accessible through study of how they interact, over time, in what we are calling the larger grammatical system. Secondly, what we might recognize as general grammatical functions also appear as rather different subsystems – modality, aspect and deixis, for example. These also may appear to interact, and, most importantly, to bear no simple relationship to well-defined structural subsystems. Thus, deixis may be studied in the personal pronoun system, in verb contrasts such as come versus go, and in the adverbials, e.g. here versus there; modality is expressed in the English auxiliary verb system but also in a subsystem of adverbials: and so on.