네가 자는 동안 노래를 만들었어

The line, explained

One person sleeps, the other stays up with a guitar. 네가 자는 동안 노래를 만들었어, 'while you were sleeping, I made you a song', is a soft little confession, and it uses a pattern that holds two actions side by side in time: -는 동안, 'while'. It tells the listener that one thing was happening during the whole stretch of another. Learn it, and you can tell the story of what you did while someone else wasn't looking.

네가 is 'you' as the subject. 너 becomes 네 before the marker 가, but because 네 and 내 (my, I) sound almost the same, many people say 니가 in speech and in songs. 자는 comes from 자다, to sleep. Its stem 자 takes -는, the ending that makes a verb describe what is going on right now, as in 자는 아이, a sleeping child. Then 동안 follows.

동안 is a noun meaning 'a period of time' or 'duration'. So 자는 동안 is literally 'the period of sleeping', which English says as 'while you were sleeping'. You can attach -는 동안 to any verb: 기다리는 동안, while waiting; 일하는 동안, while working. Stems ending in ㄹ lose it: 놀다 becomes 노는 동안. After a noun, drop -는: 여름 동안, during the summer; 3년 동안, for three years.

노래를 is 'a song' as the object. 노래 is song, and 노래하다 is to sing. 만들었어 is the casual past of 만들다, to make: stem 만들, plus -었, the past marker, plus 어. Pronounced, it flows into man-deu-reo-sseo. Notice that the past tense sits only at the end of the sentence. Even though the sleeping also happened in the past, 자는 stays in the present form, because the final verb sets the time for the whole line.

Sing it softly, as if you might wake them: ne-ga ja-neun dong-an, then no-rae-reul man-deu-reo-sseo. Now change what the other person was doing: 네가 우는 동안, while you were crying; 네가 기다리는 동안, while you were waiting. Or flip it to a promise: 내가 없는 동안 잘 지내, take care while I'm gone. That one is a farewell line you will hear in many an airport scene.