たとたん（に）the moment; just as
The very moment one thing was done, something else happened: suddenly, and not by your choice.
The moment I sat down, the phone rang.
How to form it
Verb in the た-form + とたん（に）
開けた → 開けたとたん. The に is optional and changes nothing. In writing it is also 途端.
- Before it
- Something over in a moment: 開けた, 座った, 立ち上がった, 聞いた, 見た, 着いた.
- After it
- What happened right then, outside your control: a noise, a change, a feeling, what someone else did.
- Tone
- Surprise. It caught you unprepared.
- Where you'll meet it
- Telling what happened, in conversation and in stories.
In use
The moment I got to the station, the train pulled out.
The moment I lifted the box, the bottom fell out.
The moment I saw his face, I cried with relief.
Two things to know
たとたん or たら?
When I opened the door, a cold wind rushed in.
What happened when you opened it.
The moment I opened the door, a cold wind rushed in.
In that very instant, before you were ready for it.
Not what you chose to do next, and not a request
English "as soon as" also covers the next thing you chose to do, and what you ask someone to do. たとたん covers neither.
Not said: 家に
(I took a shower as soon as I got home.)
You chose to. That is the て-form: 家に帰って、すぐシャワーを浴びた.
Not said: 駅に
(Please call me as soon as you get to the station.)
A request takes たら: 駅に着いたら、すぐ電話してください.
Close neighbours
と～た・たら～たN3
Plain "when I did, I found". Nothing in it says the same instant, and what you found may have been so for hours.
When I got to the venue, registration had already closed.
次第N2
"As soon as" for what will be done next, in business talk and announcements.
As soon as the person in charge is back, we will return your call.
Check yourself
1Which one fits?
仕事が
Choose an answer to see why.
2Which ending fits?
部屋に
Choose an answer to see why.
3Which one fits?
急に
Choose an answer to see why.
More detail
そのとたん（に） opens a new sentence: "at that very moment".
I opened the door. At that very moment, a bell started ringing.
More sentences
The moment I went outside, the rain got heavier.
The moment I turned on the light, I noticed a cat in the corner of the room.
The moment I opened the app, an error message appeared.
The moment I heard that name, old memories came flooding back.