I am grown up and I still remember that you,sister promised to let me hang out(, allow me to do whatever I want, etc.) '-단' makes words, sentences in front of this('-단') change into 'past form'. actually, '-던' is right form. and '-던' could be changed by which and how many words comes at the head of sentence. I mean it has fluid form. ex)-했던, -한다던... you can understand it as it means 'that ~~~ was ...', 'It was that ~~~~'