RM learned English by watching the show Friends
November 2017

When BTS does an English-language interview, RM is often the first to grab the mic, answer questions, and help translate for the other members along the way. He didn't learn English by living in the US or any other English-speaking country. Much of the credit, as he's told the story himself more than once, belongs to his mother, some DVDs, and one of the most famous sitcoms of all time, Friends.
American audiences got especially well acquainted with the story in November 2017, during BTS's first appearance on The Ellen DeGeneres Show. Asked how he'd learned English so well, RM answered: "My English teacher was the sitcom Friends." He explained that the show had been hugely popular in Korea when he was a kid, and that his mother bought him the DVD set of all ten seasons.
He didn't just watch them once, though. RM described the system he used: he'd watch an episode first with Korean subtitles, then with English subtitles, and finally with no subtitles at all. That way he already knew the story, but each time he could pay closer and closer attention to the words, the pronunciation, and how the characters actually talked to each other.
And his mother clearly wasn't the only one who'd landed on that idea. RM explained that Friends had a reputation among Korean parents at the time as a good way to learn English. Laughing, he said he was initially a kind of "victim" of that trend, but that thanks to it, he now considers himself lucky.
Of course, Friends wasn't the only reason behind his English skills. RM was an excellent student and also studied the language in school, further sharpening it over the years through music, movies, and everyday use. But he's singled out Friends as an important part of his learning so many times that the story has become one of the best-known details from his life before BTS.
How useful it turned out to be became especially clear once BTS started regularly appearing on American shows. As the other members gradually became more comfortable with English too, RM often carried the bulk of the conversation in the early years of their American promotions, translating questions and stepping in whenever someone got stuck. Seven members, fast-paced American interviews, and one RM in the middle of it all made for some pretty chaotic situations at times.
And it all traces back to a box of DVDs his mother bought him years earlier. She could hardly have known then that her son would one day sit in an American TV studio, explaining in fluent English to Ellen DeGeneres how Ross, Rachel, Chandler, Monica, Joey, and Phoebe taught it to him.
You can watch that very clip from The Ellen DeGeneres Show from 2017 below, where RM tells the story himself of learning English through Friends.
Watch
Ellen Makes 'Friends' with BTS! · The Ellen Show
Sources: The Ellen DeGeneres Show · BANGTANTV · BTS interviews
Unofficial fan site. Video embedded from the official YouTube channel.