～ところだった— Almost Happened

• もう少しで電車に乗り遅れるところだった。

  Mou sukoshi de densha ni noriokureru tokoro datta.

  I almost missed the train.

  ところだった shows the missing nearly happened but was avoided.

• 危うく大事なファイルを消してしまうところだった。

  Ayauku daiji na fairu o keshite shimau tokoro datta.

  I nearly deleted an important file.

  危うく + ところだった emphasizes the narrowly avoided mistake.

• あと一歩で落ちるところだったよ。

  Ato ippo de ochiru tokoro datta yo.

  One more step and I would have fallen.

  ところだった conveys the fall was imminent but did not occur.

• ・Use the dictionary (non-past) form before ところだった — the event did NOT actually happen.
• ・Commonly reinforced by もう少しで, あと少しで, or 危うく.
• ・Expresses relief about a near miss (often bad outcomes narrowly avoided).

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～ところだった 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