Language emerges from language use, as learners discover the regularities and patterns of the language in communication. In order to account for the emergence of the language, there is a need to investigate how language is formed and influenced by input over time. A group study showed that the moment at which incidental input is provided plays an important role in learning contextual vocabulary knowledge (collocations and associations). Then I intended to take a closer look at the processes of how input may shape learners' habit of using words from their writing productions. I found that each learner’s vocabulary learning process was variably dependent on the amount of input and that each individual has his/her own particular way to develop the language. I also explored the two types of language productions, writing and speaking, in lexical and syntactical performances. I found that writing has higher lexical diversity than speaking whilst speaking has higher syntactical complexity than writing, that the relation between writing and speaking changes over time, either competitive or supportive, and that the individual pattern of language development is not identical even in identical twins.