Although babies learn how to speak at different rates, almost all little ones learn how to form words and sentences in a similar order, beginning with single syllables and graduating to more complex ideas like tense. In just a few short years, a child goes from no language at all to forming cohesive sentences following grammatical rules. This process is called syntactic development.
Syntax refers to the rules used to combine words to make sentences; syntactic development is the way children learn these rules. Syntactic development is measured using MLU, or mean length of utterance, which is basically the average length of a child's sentence; this increases as a child gets older. According to Jean Berko Gleason's book, "The Development of Language," kids go through five stages of syntactic development which were identified by Roger Brown in 1973. Children automatically develop syntactic rules without explicit instruction; they learn it simply by listening to others speak around them.
Between the ages of 12 and 18 months, babies usually begin to use words to communicate, beginning with one-word utterances, such as "more," "go" or "dog." Within a few months of uttering their first words, they move into Stage I of syntactic development, two word combinations. According to Gleason, these primitive sentences mostly consist of nouns, verbs and adjectives with a lack of important grammatical elements.
As children move through the five stages of syntactic development, their sentences grow in length. According to speech language pathologist Caroline Bowen, kids begin to learn grammatical elements in Stage II, usually between 28 and 36 months. Most toddlers acquire these elements in the same order, beginning with the present progressive -ing, then the prepositions in and on.
Bowen writes that Stage III includes the acquisition of irregular past tense words, such as "fell," followed by adding "s" to possessives, then proper use of "to be" verbs, such as "are" vs. "is." This stage usually occurs between 36 and 42 months.
Stage IV, which comes between 40 and 46 months, includes understanding of articles, the regular past tense (adding -ed), and third person regular present tense, such as "He laughs." Toddlers usually apply general rules to all words before learning irregularities. For example, a toddler will often say "goed" or "foots" before he says, "went" or "feet." But this shows understanding of the rules; it's another automatically learned phenomenon.
From 42 months on, children reach Stage V, which includes using contractions, such as "I'm" and "you're." They use third person irregular present tense, such as "she has," and more complicated uses of "to be" verbs, such as combining them with other verbs and forming contractions with them. According to Bowen, kids have usually mastered all of these stages by 52 months and should be able to form four to five word sentences around age 4.