Language development
Language development is a process that starts early in human life, when a person begins to acquire language by learning it as it is spoken and by mimicry. Children's language development moves from simplicity to complexity. Infants start without language. Yet by four months of age, babies can read lips and discriminate speech sounds.
Usually, language starts off as recall of simple words without associated meaning, but as children age, words acquire meaning, and connections between words are formed. In time, sentences start to form as words are joined together to create logical meaning. As a person gets older, new meanings and new associations are created and vocabulary increases as more words are learned.
Infants use their bodies, vocal cries and other preverbal vocalizations to communicate their wants, needs and dispositions.  Even though most children begin to vocalize and eventually verbalize at various ages and at different rates, they learn their first language without conscious instruction from parents or caretakers.  It is a seemingly effortless task that grows increasingly difficult with age.  Of course, before any learning can begin, the child must be biologically and socially mature enough.
Biological Preconditions
Linguists do not all agree on what biological factors contribute to language development, however most do agree that our ability to acquire such a complicated system is specific to the human species.  Furthermore, our ability to learn language may have been developed through the evolutionary process and that the foundation for language may be passed down genetically.
Social Preconditions
It is crucial that children are allowed to socially interact with other people who can vocalize and respond to questions.  For language acquisition to develop successfully, children must be in an environment that allows them to communicate socially in that language.
There are a few different theories as to why and how children develop language.  The most popular explanation is that language is acquired through imitation.  However, this proves to be more of a folk tale than anything.  The two most accepted theories in language development are psychological and functional.  Psychological explanations focus on the mental processes involved in childhood language learning.  Functional explanations look at the social processes involved in learning the first language.
Development of English syntax Although development is highly variable, these stages based on Roger Brown's 5 stages are a useful indicator:
Stage 1: 0-26 months Present tense or present progressive (-ing) only, 1st person subject pronoun, questions signalled by intonation and later in the form "what+X+doing/going" and no/not added to sentence structure, here/there/this/that used only with gesture
Stage 2: 27-30 months Quasi-model verbs used "wanna, hafta", past tense copula (was, were) and some regular past tense verbs, 1st person object and possessive pronouns and later 2nd person pronouns, some question inversion and questions form "what/verb + V + O?", auxiliaries such as can't, don't used in negative sentences.
Stage 3: 31-35 months Future tense used, articles "a", "the", 3rd person pronouns, auxiliary form used in questions "I can't play?", later inversion occurs "Can't I play?",copula verb acquired and regular past tense "-ed" used, coordinating ans subordinating conjunctions such as "and", "or", "but" used correctly.
Stage 4: 36-40 months Plurals emerge and possessive "'s" acquired, plural pronouns, modal verbs, questions words "who, which, when how" emerge.
Stage 5: 41-46 months Irregular and regular past tense, reflexive pronouns, question tags and negative copulas used.
Stage 5+: 47+ months behind/in front, negative questions, negative pronouns "nothing, "nowhere".