나 수영할 줄 몰라!

The line, explained

The villains are closing in, the river is the only way out, and the hero's partner yells the worst possible confession: 나 수영할 줄 몰라! 'I don't know how to swim!' It is a classic action-comedy beat, and it teaches a pattern for talking about skills: -(으)ㄹ 줄 알다 and -(으)ㄹ 줄 모르다, 'know how to' and 'not know how to'. You will use it about yourself sooner than you think, perhaps the first time someone asks if you speak Korean.

나 is 'I', casual, with the subject marker dropped. 수영할 comes from 수영하다, to swim; 수영 is the noun, swimming, and 하다 makes it a verb. The stem 수영하 ends in a vowel, so it takes -ㄹ, which tucks under the last syllable: 수영할. Then comes 줄, a little noun meaning 'the way' or 'the method'. So 수영할 줄 is 'the way to swim'.

Then the verb of knowing. 알다 is to know, casual 알아; 모르다 is to not know, casual 몰라. Korean has a separate verb for not knowing, rather than putting 'not' in front of 'know', so 안 알아 is wrong. 모르다 is irregular: the 르 doubles into ㄹㄹ before the ending, giving 몰라, pronounced mol-la. Say 수영할 줄 알아, I know how to swim, or 몰라, I don't.

For consonant stems use -을 줄: 읽다 → 읽을 줄 알아, I can read. This pattern is about learned skills, the things you acquired through practice. That makes it slightly different from -(으)ㄹ 수 있다, which means 'can' in a broader sense, including whether something is possible right now. You might know how to drive, 운전할 줄 알아, but still be unable to today, 오늘은 운전할 수 없어, because you left your licence at home.

Shout it at the edge of the bridge: na su-yeong-hal jul mol-la! Then confess other missing skills: 나 운전할 줄 몰라, I can't drive; 나 거짓말할 줄 몰라, I don't know how to lie, which is a very romantic thing to say in a drama. And practise the question you are bound to hear: 한국어 할 줄 알아요? Do you speak Korean? You can now answer: 조금 할 줄 알아요, I know a little.