구나

ah, so that's how it is

Marks the moment something clicks, usually said half to yourself: 아, 그렇구나 'ah, I see', 흐름 파악이 중요하구나 'so reading the flow is what matters'. It also comments on something you've just worked out about the other person: 못했구나 'so you couldn't sleep after all', 아니구나 'oh, you're not Simsim'. It is casual — the polite version is 군요 — and unlike 네요 it sounds like a realization landing rather than a remark aimed at the listener.

Register: casual, often self-directed