내 심장이 뛰어

The line, explained

You can almost hear the drum kick under this one. 내 심장이 뛰어, my heart is racing, is the kind of line a group shouts together right as the chorus hits, usually with a choreography move clutching the chest. It is energetic, a little dramatic, and built from the most important ending in casual Korean: the plain present tense, -아/어.

내 means my, the casual form you hear in nearly every lyric. 심장 is the heart as an organ, the one that physically pounds. Korean has another heart word, 마음, which means heart in the sense of feelings and mind. When a song wants the drumbeat, it uses 심장. When it wants tenderness, it uses 마음. Here the 이 after 심장 is the subject marker, because 심장 ends in a consonant.

뛰어 comes from 뛰다, which means to run or jump, and for a heart, to beat hard. Drop 다 to get the stem 뛰, then add the casual ending. The rule: if the last vowel is ㅏ or ㅗ, add 아; for anything else, add 어. 뛰 has ㅟ, so it takes 어. The result, 뛰어, is pronounced ttwi-eo, with a tense double ㄸ at the start.

This single ending does a lot of work. It covers what is happening right now (my heart is racing), what happens regularly (my heart races whenever I see you), and with a rising voice, questions (is your heart racing?). Sometimes vowels merge: 가 plus 아 stays 가, and 보 plus 아 becomes 봐. Any verb built on 하다 simply becomes 해. Learn these and you can already chat.

This is 반말, the casual speech used between friends, with family and in almost all lyrics. To make any of these polite, just add 요: 뛰어요, 가요, 해요. You will hear that version in variety shows when idols speak to hosts. Try swapping the verb in this line: 내 심장이 떨려 means my heart is trembling, and 내 심장이 아파 means my heart hurts.