ㄴ단

Shortened 'that', in 단 말이야

The same construction as 단, spelled differently because it attaches to present-tense action verbs. Like 단, it lives inside one phrase: all eight lines here follow it with 말 or 거. 단 말이야 means roughly 'I'm telling you' or 'the thing is', pressing your point and asking the listener to take it on board. Learn the whole phrase rather than the ending alone, since on its own it will rarely make sense.

Register: conversational