〜たとたん（に）
The moment 〜たとたん
Meaning
"The moment ~", "just as ~" — the second event follows instantly and unexpectedly.
Formation
[verb, た-form] + とたん（に）
Notes
Takes the た-form only. The second clause must be something that simply happened — a surprise, not a plan — so it cannot be a request, an intention or a command: ×立ったとたんに座ってください. Usually reports the past.
Examples
ドアを開（あ）けたとたん、猫（ねこ）が飛び出（とびだ）した。
The moment I opened the door, the cat shot out.
Breakdown
立ち上（たちあ）がったとたんにめまいがした。
The instant I stood up I felt dizzy.
Breakdown
外（そと）に出（で）たとたん、雨（あめ）が降り出（ふりだ）しました。
Just as I stepped outside, it started to rain.
Breakdown
彼（かれ）の顔（かお）を見（み）たとたん、名前（なまえ）を思い出（おもいで）した。
The moment I saw his face, I remembered his name.
Breakdown
電車（でんしゃ）に乗（の）ったとたんに眠（ねむ）くなった。
The moment I got on the train I felt sleepy.
Breakdown
⚠️ Often confused with
〜た後で / 〜後
After doing 〜後で
"After doing ~". Attach to the ta-form of a verb, or a noun + の. Note the contrast with...
〜てから
After doing (and then) 〜てから
"After doing ~ (and only then ~)". Emphasizes that action B happens AFTER action A is comp...
〜前に
Before doing 〜前に
"Before doing ~". Attach to the dictionary form of a verb or a noun + の. The action AFTER...
〜次第
As soon as 〜次第
"As soon as ~", "the moment ~ is done". Formal, and always about the future.
〜以来
Ever since 〜以来
"Ever since ~" — a state that has held continuously from a past point until now.
〜たびに
Every time 〜たびに
"Every time ~", "whenever ~" — the second thing happens on each occasion of the first.
〜ぶり
After an interval 〜ぶり
"After an interval of ~" — how long it has been since this last happened.
〜かと思うと
No sooner than 〜かと思うと
"No sooner had ~ than ~", "just when you think ~" — two events tread on each other.
〜上で
After (formal) 〜上で
"After ~ing (and on that basis)" — a deliberate step taken before deciding.
〜ところに / 〜ところへ
Just as / right when 〜ところに
"Just as ~", "right when ~" — something arrives at exactly that moment.
〜に際して
At the time of 〜に際して
Formal "on the occasion of ~", "at the time of ~" — used as something significant begins.
〜てはじめて
Only after 〜てはじめて
"Only after ~", "not until ~ did I …".
〜てからというもの
Ever since then 〜てからというもの
"Ever since ~" — a turning point after which things have stayed changed.
〜に先立って
Prior to 〜に先立って
"Prior to ~", "in advance of ~" — a preliminary step before a major event.
〜際（に）
On the occasion of 〜際（に）
A formal "when ~", "on the occasion of ~".
〜が早いか
The instant that 〜が早いか
"The instant ~ happened, ...", "no sooner ~ than ...".
〜や否や
Scarcely when 〜や否や
"As soon as ~", "scarcely had ~ when ..." — the two events are almost simultaneous.
〜そばから
No sooner done than undone 〜そばから
"No sooner is ~ done than (it is undone)" — the effort is immediately cancelled out.
〜なり
Immediately upon 〜なり
"The moment ~", "immediately upon ~ing" — one action leads straight into an unexpected nex...
〜を境に
Marking the turning point 〜を境に
"With ~ as the dividing line", "ever since ~" — a clear before and after.
〜がてら
While also doing 〜がてら
"While ~ing", "taking the opportunity of ~ing to also ..." — two purposes in one outing.
〜間際に
On the verge of 〜間際に
"Just as ~ is about to happen", "at the very last moment before ~".