Japatan
Formation
- Vた形 + とたん(に)
Explanation
AたとたんB means that B happened immediately after A. B is something the speaker did not expect, often surprising. Because it describes an observed event, the main clause is usually in the past tense and cannot express the speaker's intention, a request, or an order. A is always in the た-form.
Examples
家を 出たとたん、雨が降りだした。
The moment I left the house, it started to rain.
急に 立ち上がったとたんに、頭が痛くなった。
As soon as I stood up suddenly, I got a headache.
先生が教室に 入ってきたとたん、みんな静かになった。
The moment the teacher came into the classroom, everyone went quiet.
窓を 開けたとたん、強い風が入ってきた。
As soon as I opened the window, a strong wind blew in.
Notes
駅に着いたとたん、電話してください is wrong. For requests or plans, use 〜たらすぐ instead.
Related grammar
Mini quiz
1ドアを（ ）とたん、ねこが外へ走っていった。
Answer and explanation
Answer: 2 開けた
"The moment I opened the door, the cat ran outside." とたん always follows the た-form, so 開けたとたん is correct.
2「たとたん」の使い方が正しい文はどれですか。
Answer and explanation
Answer: 3 ベッドに横になったとたん、寝てしまった。
Sentence 3, "I fell asleep the moment I lay down on the bed," is correct. 1 expresses intention and 2 is a request, which とたん cannot take. 4 uses the dictionary form instead of the た-form.
3試験が終わった（ ）、教室が急ににぎやかになった。
Answer and explanation
Answer: 4 とたん
"The moment the exam ended, the classroom suddenly became noisy." とたん fits. 最中に and うちに cannot follow a finished action, and ついでに is for doing an extra task.
