ㄴ가 달라지다

something feels different now

뭔가, or the fuller 무언가, is 'something, you can't say what', and 달라지다 is having become different. Every line here is that pair: something shifts as your experience builds, something changed over the last five years, his expression changed. You use it exactly when you can feel the change but cannot put your finger on it.

Register: casual, reflective