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There is an interesting situation here which would be a paradox unless we were willing to make assumptions on the nature of the learner: as investigators of the nature of language, it is preferable to concentrate on understanding; the objects that are to be understood are sentences; the sentences that are actually heard are frequently “degraded specimens ’’from a grammatical point of view-they are semisentences at best; on the other hand, the understanding of semisentences is apparently more difficult proper sentences. We first seem to learn the rules and principles underlying grammatically correct sentences, and only by virtue of having acquired these can we begin to understand semisentences. (This becomes particularly evident in the acquisition of a second language by a good lecturer or presented in print, long before we can understand a conversation which is heavily loaded with semisentences ) . The paradox is this: if the child’s task is to abstract principles that generate correct sentences, but is presented indiscriminately with semi- and proper-sentences, how can the correct principles be established, and why or how does his understanding of sentences become fairly explainable in terms of a grammatical theory? The assumption that we have to be willing to make here concerns the cognitive machinery that we must suppose to be developing in the child.
If the most promising source-material for a theory on syntacticmechanisms is understanding, what data should we use to construct a theory on the development of syntax in children? Preferably, the child’s development of understanding. His actual utterances may, in certain cases at least, be irrelevant to his development of syntactic mechanisms(for instance, in children with severe psychiatric disease who may not choose to speak, or who prefer to make animal noises, or in children whose noises cannot be understood). By and large it is true that young children can understand more than they can say.＊
Children between 18 and 36 months seem to have a tendency to run constantly through their repertoire of capacities. This is also reflected in their verbal behavior in that during this period the gap between their understanding and speaking capacity normally remains fairly constant and predictable. This may be tested by asking them on the one hand to execute certain verbal commands or to point to pictures that are being described to them in more or less complex sentences, and on the other hand to require them to repeat accurately sentences that are given them.
Since a sentence contains so much detail we cannot repeat it correctly upon a single presentation unless we can apply grammatical principles to it by means of which the mass of information can at once be recoded and thus processed in much simplified from (Mehler and Miller, 1964). The utterances of a child who is just beginning to speak (normally not much later then 30 months) may thus reflect the stages that his development of language capacities, particularly understanding have traversed, even though one may actually have taken place some 2 months before the other. By about 30 months, however, production soon becomes as unreliable an indicator of language capacities as is the case in the adult. Unfortunately, no studies have yet been published that have undertaken systematic research on the development of grammatical understanding of the child at this age and older. Even the best studies have relied too heavily on production.
(b)How Mature Speakers Understand Sentences. Some insight into this problem is provided by asking ourselves why a sentence such as
They are boring students has two meanings. Here the explanation is quite simple; we may choose to link the word boring to the word are as in Fig.
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 But Roger Brown (personal communication), in his extensive investigations of the first steps toward language acquisition, has found that this is not always and necessarily true. For example, there were instances in his sample in which plural inflections were used productively at time when experiments on the child’s semantic progress indicated that he did not yet know what this particular suffix signals. Similar observations were described by Fraser et al. (1963).