〜たとたん（に） means "the very instant that X happened, Y (suddenly, unexpectedly) happened." Two completed events, snapped together with no gap — but とたん adds something the plain word "when" doesn't: surprise. The second event isn't the natural next step in a plan; it ambushes the subject. You stand up and — bang — the dizziness hits; she sees your face and — before she can stop it — bursts into tears. Because Y is characteristically sudden, involuntary, and outside anyone's control, とたん lives entirely in the past and can never introduce a command, a request, an invitation, or an intention. This is the grammar of the unforeseen hinge: the moment one thing ends, another thing you didn't see coming swings into place.
The shape: past-tense verb + とたん(に)
The single most important formation rule is that とたん attaches to the past plain form (the た-form) of a verb — because X is a completed event.
[ verb-た ] とたん（に）、[ sudden past event ]
家に着いたとたん、雨が降り出した。
ie ni tsuita totan, ame ga furidashita
The instant I got home, it started to rain.
私の顔を見たとたん、彼女は泣き出した。
watashi no kao o mita totan, kanojo wa nakidashita
The moment she saw my face, she burst into tears.
立ち上がったとたんに、めまいがした。
tachiagatta totan ni, memai ga shita
The instant I stood up, I felt dizzy.
The に is optional (とたん and とたんに are both fine; とたんに is a shade more emphatic). The word is written 途端 in kanji but very often appears in kana as とたん. Note that both clauses describe finished, past-tense events — 着いた…降り出した, 見た…泣き出した — which is the norm for this pattern.
Why Y must be involuntary and surprising
Here is the insight that makes every rule below predictable. とたん doesn't just mark simultaneity — it dramatizes it. The literal 途端 evokes an "edge / brink," and the construction frames the second event as tumbling off that edge the instant the first one ends. That framing carries an emotional colour: the speaker was not expecting it, or could not have stopped it. So the second clause is overwhelmingly a spontaneous physical or emotional event: 泣き出す (burst into tears), 降り出す (start pouring), 倒れる (collapse), 眠ってしまう (fall asleep). You'll notice how often these verbs are themselves "sudden-onset" compounds in 〜出す / 〜始める.
ドアを開けたとたん、猫が飛び出してきた。
doa o aketa totan, neko ga tobidashite kita
The instant I opened the door, the cat came darting out.
電気を消したとたん、赤ちゃんが泣き始めた。
denki o keshita totan, akachan ga nakihajimeta
The moment I turned off the light, the baby started crying.
彼は椅子に座ったとたん、いびきをかき始めた。
kare wa isu ni suwatta totan, ibiki o kakihajimeta
The instant he sat down in the chair, he started snoring.
名前を呼ばれたとたん、緊張で頭が真っ白になった。
namae o yobareta totan, kinchō de atama ga masshiro ni natta
The instant my name was called, my mind went blank with nerves.
In every one of these, the subject is a passenger, not a driver. That is the emotional signature of とたん.
とたん vs たら / と: controllable vs. ambushing
This is the distinction English speakers most need, because たら and と also translate as "when / and then." The difference is control.
- たら and と can introduce a planned, controllable follow-up: "when you get to the station, call me." The speaker chooses what happens next.
- とたん foregrounds instantaneous, involuntary succession with a jolt of surprise. The subject didn't choose Y; Y happened to them.
ニュースを聞いたとたん、彼女は言葉を失った。
nyūsu o kiita totan, kanojo wa kotoba o ushinatta
The moment she heard the news, she was lost for words.
春になったとたん、公園の桜が一斉に咲いた。
haru ni natta totan, kōen no sakura ga issei ni saita
The instant spring arrived, the cherry blossoms in the park all bloomed at once.
There is a genuine overlap: たら can also report a surprise discovery (the "たら of discovery", 帰ったら手紙が届いていた). But even there, たら is broader and calmer; とたん zooms all the way in on the split-second hinge and insists on the drama. If you want a neutral "when," reach for とき instead — it carries no surprise and no instantaneity.
The literary cousins: 〜が早いか and 〜なり
Two stiffer, more literary patterns express a similar "the moment…" and are worth recognizing even though you'll rarely produce them in speech. Both attach to the dictionary (non-past) form.
彼は玄関に入るが早いか、二階へ駆け上がった。
kare wa genkan ni hairu ga hayai ka, nikai e kakeagatta
No sooner had he stepped into the entryway than he dashed up to the second floor. (literary)
彼女は席を立つなり、何も言わずに部屋を出て行った。
kanojo wa seki o tatsu nari, nani mo iwazu ni heya o dete itta
The moment she rose from her seat, she left the room without a word. (literary)
〜が早いか and 〜なり (both literary) belong to novels and formal narration; in everyday conversation, とたん is the natural choice. Keep them for reading, and note their surprising formation — they take the plain non-past even though the event is completed, whereas とたん takes the past た-form.
Common mistakes
Mistake 1 — Attaching it to the non-past form. The headline error: putting a dictionary-form verb before とたん. It must be the past た-form.
❌ ドアを開けるとたん、猫が飛び出した。
Wrong form — とたん needs the past た-form: 開けたとたん. (開けるとたん is ungrammatical.)
✅ ドアを開けたとたん、猫が飛び出した。
doa o aketa totan, neko ga tobidashita
The instant I opened the door, the cat leapt out.
Mistake 2 — Putting a request or command after it. Because Y is involuntary and past, an instruction cannot follow. Use たら.
❌ 駅に着いたとたん、電話してください。
Broken — 'please call' is a request the listener controls, which とたん forbids. Use たら: 駅に着いたら、電話してください。
✅ 駅に着いたら、電話してください。
eki ni tsuitara, denwa shite kudasai
When you get to the station, please call me.
Mistake 3 — Attaching a volitional or future intention. とたん can't carry "I will / I intend to." That, too, is たら territory.
❌ 家に帰ったとたん、すぐ宿題をするつもりだ。
Wrong — つもりだ is an intention, which the ambushing とたん can't host. Use たら: 家に帰ったら、すぐ宿題をするつもりだ。
✅ 家に帰ったとたん、どっと疲れが出た。
ie ni kaetta totan, dotto tsukare ga deta
The instant I got home, a wave of exhaustion hit me.
Mistake 4 — Using it for a slow or gradual result. とたん demands instantaneity. If Y unfolds over time, it's the wrong connector.
❌ 薬を飲んだとたん、少しずつ元気になった。
Wrong — 少しずつ (little by little) is gradual, which contradicts the split-second とたん. Use と or たら for gradual change.
✅ 薬を飲んだとたん、急に眠くなった。
kusuri o nonda totan, kyū ni nemuku natta
The instant I took the medicine, I suddenly got sleepy.
Key takeaways
- 〜たとたん（に） = "the very instant X happened, Y suddenly happened" — attach it to the past た-form, never the dictionary form.
- Y must be past, instantaneous, and involuntary — a reaction, an onset, a mishap that ambushed the subject. It cannot be a command, request, invitation, intention, or gradual change.
- The emotional signature is surprise: the subject is a passenger, not a driver (note how often Y is a 〜出す / 〜始める "sudden-onset" verb).
- For a controllable or requested next step use たら; for a neutral "when," use とき.
- The literary 〜が早いか and 〜なり express a similar "the moment…" but are stiff, book-only, and attach to the non-past form.
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