Human beings refer to communicate with each other to share our knowledge so it means that using language to share the knowledge with individuals make human different than others in the universe. It is stated that ' 'Language can be defined as a socially shared code or conventional system for representing concepts through the use of arbitrary symbols and rule-governed combinations of those symbols''. (Owens, 2012) Language is not just a term refers to norms of speech; it is also related with the culture of different societies. Language has been viewed ' 'as a complex communication system which must be analyzed on a number of levels: Phonology, syntax, morphology, lexis, semantics, pragmatics, discourse.''. (Mitchell, Myles & Marsden, 2013). .
Languages, like human, ' 'evolve, grow and change.''. When we look at the papers of the history, it can be seen that there are a lot of language which has been changed or lost. Linguists, who focus on the language and its' components, indicated that some languages still endangered as it is stated that 'Languages face extinction as surely as plants and animals'. (Owens, 2012).
There are basic terms about language that should be known before look at the stages of second language acquisition, like bilingual, multilingual, first language acquisition and second language acquisition. The term of bilingual refers to 'someone who speaks two languages', on the other hand the term of multilingual refers to 'someone who speaks more than two languages'. (Birner, 2013). In opposites with bilinguals, monolingual refers to someone who uses only one language. (Troike-Saville, 2006). First language acquisition is the study which focuses on the infants' acquisition of their native languages on the other hand second language acquisition deals with the acquisition of additional languages and it is 'the study of how second languages are learned.