The Acquisition of Grammatical Morphemes. We have noted that Stage I children's speech has been called "telegraphic," since it includes contentive words but not function words or inflections. Stage II (defined by a MLU of 2.00 to 2.50) is characterized by the appearance of some of the function words and inflections. These do not have meanings in isolation, but modify the meanings of Stage I speech. They add such things as number and tense, mark whether nouns are specific ("the book") or nonspecific ("a book"), designate distinctions such as that between in and on, and play other such roles.

Brown (1973) writes that what is acquired in this stage is knowledge of the "grammatical morphemes and the modulations of meanings." He discusses 14 grammatical morphemes, and presents evidence that the order in which the child begins to use them appropriately is determined by the semantic and grammatical complexity of the morphemes, not by how frequently the child hears them. The complexity measures are based on linguistic analysis and we will not get involved with them here. Brown's data in support of this claim come largely from corpora of the spontaneous utterances of several children. It has been shown that comprehension generally precedes production (Fraser, Bellugi and Brown, 1963); children in experimental situations show some understanding of the grammatical morphemes (e.g., picking the picture of "the cows" as "the cows" correctly) before they use them in spontaneous or elicited speech.

Berko (1958) did an important study of children's use of the inflections marking plurals, possessive past tense, present progressive (e.g., hitting), and third person singular (hits) verbs. This study clearly showed that children induce rules for using these, since they were able to apply them to made up words to produce such forms as "wugs", "zebbing", "bik's", "spowed", and "lodges". Berko used a technique of elicited productions. For example, she would point to a picture of a funny, bird-like creature and say to the child: "This is a wug." She would then point to a picture of two of them and say: "Now there are two of them. There are two?" If the child supplies the missing word with a plural marker ("wugs"), as most of the children did, he has mastered the rules for forming plurals. The same induction of rules is obvious in spontaneous speech when the child over-generalizes his use of the rules and produces forms he probably never heard, such as "sheeps," "comed," "gived".

More Advanced Syntax. The rules for plurals, past tense, and so forth, are relatively simple. The rules for characterizing all the knowledge of the language an adult knows are very complex; no linguist has yet been able to describe them completely. Chomsky (1957, 1965) has attempted to write a set of rules which designate how all grammatical sentences of English can be "generated" or built up by a formal, step-by-step procedure. Two types of rules are used: "rewrite" or "phrase-structural" rules, which operate hierarchically and generate a base or "deep" structure; and "transformational" rules, which operate on the entire deep structure and transform it into the actual sentence or "surface" structure. Transformational rules enable linguists to account for the similarities among active, passive, negative, question, and other forms of the same basic sentence. Transformational rules also enable embedded sentences and conjunctions to be generated.