
Well, Hangyum told Hajin that the one who went to school with him was his brother.. In other words, he says that Hangyum is actually Hanwool and vice versa.. Hajin believed his words but then (as you saw in the latest chaps) he meets Hanwool, again, with Sohyun and she introduces him as Hanwool, Hangyum's twin, to Hajin.. That's how he knew that both the brothers lied to him..

Yes. Go to chapter 28. Find the title. The third picture down from the title. Second bubble. They did not talk much and I did not think Hanwool did a good job being Hangyum. I always thought Hajin called him out on it because of his last reaction and it never went back to them after the flashback. Also, he started to act a little different toward Hangyum. But I guess he just started to believe the lie?
I’m kinda confused so how did Hajin end up figuring out it was Hagyum all this time? I thought when they went back to the school, Hajin still thought he was his brother