웃든지 울든지 오늘은 네 맘대로 해

The line, explained

Some lyrics give orders and some give permission. Today's is the second kind. The singer is telling someone that tonight, whatever they feel, it is fine, and the grammar that carries that easygoing shrug is -든지. You will hear it constantly in everyday Korean, usually in pairs, and once you know it you will spot it in phrases you may already have memorised, like 언제든지 (anytime) and 뭐든지 (anything).

The first two words are a matched pair. 웃든지 is 웃다, 'to laugh' or 'to smile', with -든지 attached to the stem 웃-. 울든지 is 울다, 'to cry', with the same ending. On their own each means 'whether you laugh', 'whether you cry'; placed side by side they make 'laugh or cry, either way'. The ending never changes shape. There is no 으 to add and no vowel harmony to worry about, which is why beginners can use it right away.

오늘은 is 오늘, 'today', plus the topic marker 은. The topic marker adds a little spotlight: as for today, at least today, as if other days might have rules but this one does not. In lyrics, 오늘 often stretches to cover the night as well, which is why the English translation says tonight. A Korean listener would hear the same mood either way, the sense of one special stretch of time set apart.

네 맘대로 해 is a phrase worth learning whole. 네 means 'your', 맘 is the casual short form of 마음, 'heart, mind', and 대로 means 'in accordance with, as'. 맘대로 is therefore 'as your heart wants', and 해 is the casual 'do'. Together: do as you please. Said sharply it can mean 'fine, suit yourself'; sung warmly, as here, it means 'you are free'. The tone, not the words, decides which.

Now play with the pair. 춤추든지 노래하든지 (dance or sing), 자든지 놀든지 (sleep or party), 먹든지 마시든지 (eat or drink). Add a question word and -든지 widens to 'any': 뭐든지 해 줄게 (I'll do anything for you), 어디든지 갈게 (I'll go anywhere). In quick speech the ending shortens to -든, so you might hear 웃든 울든. Just keep it apart from -던지, a different ending about memories, which looks almost the same on paper.