たとたん（に）The instant that
たとたん（に） says that something happened the instant another action was completed. The second event is typically sudden, unexpected, spontaneous, or outside the speaker's control.
An exact moment of change
The first action must have a clear endpoint: opening something, sitting down, switching something on, arriving, and so forth. The second event is presented as occurring right at that endpoint, often before anyone has time to prepare or respond. In general writing, 途端 is also a common spelling.
Typical sudden results
The result may be a physical reaction, an outside event, or a discovery made at that exact moment.
熱いスープを飲んだとたんに、せきが出た。
The instant I drank the hot soup, I started coughing.
AnalyzeThe cough is an involuntary physical reaction.
ドアを開けたとたん、冷たい風が吹き込んできた。
The instant I opened the door, a cold wind rushed in.
AnalyzeThis version omits the optional particle.
電気をつけたとたん、部屋の隅に猫がいるのに気づいた。
The instant I turned on the light, I noticed a cat in the corner of the room.
AnalyzeTurning on the light creates the exact moment at which the discovery occurs.
Choosing among similar time expressions
たとたん fixes the second event to the exact moment the first action finishes. かと思うと and かと思ったら instead highlight an observer's surprise at a rapid change or reversal. The pattern か ... ないかのうちに goes further by suggesting that the second event begins before the first can even be confirmed as complete. A discovery sentence using たら simply reports what was found after an action and does not by itself require an immediate or surprising result.
More examples
席に座ったとたんに、電話が鳴った。
The instant I sat down, the phone rang.
Analyze外に出たとたん、雨が激しくなった。
The moment I went outside, the rain grew heavier.
Analyzeその名前を聞いたとたん、昔の記憶がよみがえった。
The instant I heard that name, old memories came flooding back.
Analyze駅に着いたとたんに、電車が出てしまった。
The instant I arrived at the station, the train pulled away.
Analyzeアプリを開いたとたん、エラーメッセージが表示された。
The moment I opened the app, an error message appeared.
Analyze箱を持ち上げたとたん、底が抜けた。
The instant I lifted the box, the bottom gave way.
Analyze彼の顔を見たとたんに、安心して涙が出た。
The instant I saw his face, tears came from relief.
Analyze