～たとたん(に)— The Very Instant

• 家を出たとたんに、雨が降り出した。

  Ie o deta totan ni, ame ga furidashita.

  The instant I left the house, it started to rain.

  出た (past of 出る) + とたんに links to an unexpected immediate event

• 彼は私の顔を見たとたん、泣き出した。

  Kare wa watashi no kao o mita totan, nakidashita.

  The moment he saw my face, he burst into tears.

  見た + とたん shows a spontaneous reaction right after seeing

• 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.

  立ち上がった + とたんに connects standing to a sudden, uncontrolled sensation

• ・Must use the past た-form before とたん.
• ・The second clause describes something sudden/unexpected and is normally past tense; avoid commands or intentions.
• ・Register: neutral; very common in narration.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～たとたん(に) is tested against.

• ～ないうちに — Before It Happens
• ～上(で) — After Doing / In the Process Of
• ～から～にかけて — Spanning From To
• ～際(に) — On The Occasion Of
• 最中に — Right in the Thick of It
• 次第 — The Moment It's Done
• ～て以来 — Ever Since Then
• ～ところに／～ところへ — Right At That Moment