The thing I find difficult about 존댓말 and 반말 is not its actual usage, but the emotional component that comes with it.
For example, I tend to slip into 반말 when I feel an emotional bond with that person, even if s/he has not ‘officially’ given the go-ahead to use 반말. Thankfully my friends are pretty understanding and do not that that I’m being intentionally rude when I make the slip. Similarly, when talking to a 친구 or 동생, I sometimes subconsciously add -요 at the end of my sentences if we are not that close.
I guess this is how Korean is very much an emotional language for me. So instead of speaking to the other party based on his or her age or status, my natural reflex is to use the form (반말 or 존댓말) that reflects how I feel towards the other party.
I remember receiving a call from an 언니 that I’m not very close to, but who had suggested that we speak informally, and somewhere at the beginning of the convo I blurted out “아, 네네.” This 언니 then said, “어, 너 왜 갑자기 존댓말을 하냐?” I must admit that I was a little embarrassed at that time. ^^;;; This actually happened last year, my 5th year into this korean language journey, even though I have been actively using Korean almost daily over the past 2 years despite the fact that I have already left Korea.
It’d be great if I could find a way to mentally “compartmentalise” people into the 반말 and 존댓말 categories, so that the appropriate speech form comes to me naturally, regardless of how close I feel to them emotionally.
Does anyone else face the same problem? ^^;: