Introduction Second language acquisition has long been the focus of linguists and researchers in teaching field. It highlights the process and theories of acquiring second languages among learners. Second language acquisition seeks to understand the development of second language acquisition among second language learners. Mastering the knowledge of second language acquisition helps teachers to successfully provide effective teaching for students. Undeniably, knowledge on second language acquisition is very much related to effective teaching of second languages. Many linguists and teacher educators believe that all language teachers should be equipped with knowledge of second language acquisition to help them teach effectively. Additionally, most teacher training programmes have incorporated second language acquisition as a compulsory course to make sure that all future language teachers are well-equipped with enough knowledge to teach in classroom. Then again, the relationship between second language acquisition and effective second language teaching is still being discussed. Hence, this paper aims to explain ways in which knowledge of second language acquisition helps language teachers to provide a better and effective second language teaching in classrooms by highlighting three main ideas; knowledge of second language acquisition helps teachers to understand the different needs of students, to design a good second language course and effective teaching method for learners, and to understand the role of motivation in second language acquisition and learning Understanding the Different Needs of Students Understanding students’ needs is very crucial for an effective second language teaching. Knowledge of second language acquisition helps language teachers to understand the different needs among students. Every student definitely has different needs according to their first language, age, and cognitive ability. In the field of second language acquisition, linguists and researchers have been arguing on the effects of learners’ first language on their second language acquisition. Based on their research on the influence of first language on second language performance, Kesumawati, Nor Zakiah, Norsimah and Nor Hashimah (2006) have reported that syntactical difference among the first language and the second language has found to be one of the major factors in students’ failure to master the second language. Since most learners have been growing up with their first language, Beardsmore (1982) then argued that second language learners are struggling with the vocabulary, phonology and grammatical structure of second language due to the interference of habit from the first language. Bhela (1999) then suggested language teachers to consider the relationship between students’ first language and the second language. He has concluded that “languages with more similar structures [eg English and French] are more susceptible to mutual interference than languages with fewer similar features [eg English and Japanese],” (Bhela, 1999, p.23). Hence, it is suggested for teachers to expect more learning difficulties among students whose first language and second language’s structure are greatly different. As a language teacher myself, I notice that the interference of first language does affect the acquisition and learning of second language in the classroom. The different structure of the two languages is not the only problem in second language classroom. In fact, the context and culture of the first language also affect students’ second language learning. Based on my teaching experience, most students failed to use the second language in a correct context and structure as th