내 너를 믿었거늘 어찌 이럴 수 있느냐

The line, explained

Historical films set in the Joseon dynasty speak a Korean that sounds grand even to native ears. Characters use endings that died out of conversation long ago, and -거늘 is one of the most powerful. It sets up a fact, something that should have guaranteed a certain outcome, and then delivers the blow: and yet this happened. Today's line is spoken by a lord to a retainer who has betrayed him. It is a reproach and a lament at once.

내 is 'I', but used here in an old way: as a subject on its own, without the 가 that modern 내가 needs. In historical dialogue, 내 standing alone at the start of a sentence sounds commanding and personal. 너를 is 'you' with the object marker 를; modern speech would often shorten it to 널, but period dialogue keeps it full and deliberate. Together 내 너를 sets up a relationship: me, and you.

믿었거늘 is the centre. 믿다 means 'to trust, to believe', and 믿었- is its past. Then -거늘 attaches directly. The ending means something like 'given that, despite the fact that', and it always looks forward to a contradiction. Pronounced, the ㅆ of the past tense becomes a stop and tenses the ㄱ, so you hear something like mi-deot-kkeo-neul, romanised mi-deot-geo-neul. The fact is stated; the betrayal is coming.

어찌 이럴 수 있느냐 is the betrayal, phrased as a question. 어찌 is a literary 'how, why'. 이럴 is from 이러다, 'to act like this', and -ㄹ 수 있다 means 'can'. 있느냐 uses -느냐, an old-fashioned question ending that sounds stern, even superior: it is how a king or elder speaks to someone below them. The ㅆ before ㄴ becomes ㄴ, giving in-neu-nya. The whole line: I trusted you, how could you do this?

Swap the first verb and the relationship changes: 내 너를 아꼈거늘 (I cherished you), 내 너를 키웠거늘 (I raised you), 내 너를 거두었거늘 (I took you in). In modern Korean you would say 내가 너를 믿었는데 어떻게 이럴 수 있어?, which means the same with none of the grandeur. You will not use -거늘 in a text message, but once you recognise it, every historical film's great confrontation will sound sharper.