～か～ないかのうちに— The Very Instant

• ベルが鳴るか鳴らないかのうちに、生徒たちは教室を飛び出した。

  Beru ga naru ka naranai ka no uchi ni, seitotachi wa kyoushitsu o tobidashita.

  The students dashed out of the classroom the instant the bell rang.

  鳴る (ring) か 鳴らない かのうちに — the running-out happened right as the bell rang, almost before it finished.

• 彼は席に着くか着かないかのうちに、もう寝てしまった。

  Kare wa seki ni tsuku ka tsukanai ka no uchi ni, mou nete shimatta.

  He fell asleep the moment he sat down.

  着く (sit) か 着かない かのうちに marks the split second of sitting down.

• 映画が終わるか終わらないかのうちに、彼女は泣き出した。

  Eiga ga owaru ka owaranai ka no uchi ni, kanojo wa nakidashita.

  She burst into tears just as the movie was ending.

  終わる (end) か 終わらない かのうちに — the crying came right at the edge of the ending.

• ・Uses the same verb repeated: dictionary form + か + ない-form + か.
• ・The main clause is normally in the past tense, describing a sudden follow-up event.
• ・Register: slightly literary/written; common in narration.
• ・Cannot be used for your own planned or willed actions — it describes spontaneous, observed events.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～か～ないかのうちに 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