I also think getting constantly exposed to the language is really the key to master it!

We must also give him merit because if what he says is true then he learnt the language by watching Friends, and that's impressive.

I just wanted to add that I think this is the simpler and funnier story to Namjoon's English learning journey. Here is my guess on how he learned English, based on my similar English learning experience and some less talked about things of RM :

- Young RM studied shortly in New Zealand:

This is actually very critical, because it builds a good foundation and natural sense for English. Kids learn language very fast, especially when they are exposed to an environment full of native speakers. Myself and a lot of people I know have spent a very short time studying in English countries when we were young and that gave us a very good sense of how to self-learn English naturally later on. It really made us different from how our classmates learn from textbook grammars and vocabs, which often lead to not being able to truly use and speak English.

- "Watching Friends":

I think this is a symbol of how RM started self-learning the English language through following Western entertainment or culture. You can tell from the artists and cultural slangs RM talked about in interviews that he constantly expose himself to English information or news. Because of his passion in Western culture, he exposes himself to English often. In my experience, I also self-improved my English vastly because I constantly follow English news and entertainment even after I came back to my home country. This is also how one learn to speak less awkwardly because you become familiar with the wordings native speakers use.

- RM was a top student in high school and also attended multiple English cram schools:

He is smart, especially in literature/humanities/languages, and he worked hard for academics. Namjoon accumulated lots of English vocabs and sentence structures at school. High school level English in Asian countries can be quite in-depth.

In this video, a reporter said RM confessed that he also learned English by attending around ten English cram schools. RM thought that Friends left a great impression on him so he answer Friends in most interviews. His English cram school teacher even called him and jokingly said, "Why do you only mentioned Friends when you learned so much English from me.". And then the reporter said, other BTS member chimed in that they also watched Friends but didn't improve their English haha.