哦

Falling-tone 哦 marks the moment the penny drops — 'ah, I see / oh, got it.' Pitch contour does all the work: drop from high to low as you realise the answer. Pair it with 原来 ('so that's why') or 知道了 ('I get it now') for the canonical 'lightbulb' reaction.

Three readings to keep straight, all written 哦: rising 哦 (doubt — 'oh? really?'), falling 哦 (realization — 'ah, I see'), and toneless sentence-final 哦 (friendly softener — 'alright?'). The falling and rising readings both stand alone at the START of a sentence; the toneless particle sits at the END. Match pitch to emotion or the same line will sound wrong: a flat 哦 in answer to news can read as dismissive 'whatever,' while a proper falling 哦 sounds engaged and warm.