The rapid development of languages around the world proves that civilization requires humans to always adapt quickly in a short time. A very interesting example of the adaptation process is the development of the use of English in Indonesian society. We all realize that English is now an international language, which means that almost all humans in the world speak English, even if it is only limited to basic knowledge. So, in the process, what is the benchmark of the use and mastery of English for people who make it a second language? In this book, the role of mother tongue in the understanding and mastery of English for Indonesians will be explained and the influence of English as a second language for Indonesians will also be discussed.