間に(あいだに) — A Point Within a Window

• 寝ている間に雨が降った。

  Nete iru aida ni ame ga futta.

  It rained at some point while I was sleeping.

  間に + 降った: the rain happened once during the sleep, not for its whole length.

• 母が買い物に行っている間に、ケーキを作りました。

  Haha ga kaimono ni itte iru aida ni, keeki o tsukurimashita.

  I made a cake while my mother was out shopping.

  行っている間に + 作りました: the cake was made at some point within the shopping window.

• 授業の間に、雨がやみました。

  Jugyou no aida ni, ame ga yamimashita.

  The rain stopped at some point during class.

  授業の間に + やみました: a one-time event within the class period, not throughout it.

• ・Adding に pins the second clause to a moment inside the window, so it should be a completed/instant action (降った, 生まれた, 盗まれた).
• ・Compare: 'I slept through the whole movie' → 間; 'I fell asleep during the movie' → 間に.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones 間に is tested against.

• 間 (あいだ) — Throughout a Duration
• あとで — After (with a time gap)
• 頃 / ごろ — Around / About (approximate time)
• までに — By / No later than (deadline)
• 〜おきに — The Gap Logic (Intervals)
• たばかり — Just did (subjective recency)
• たところ — Just finished (this very moment)
• ているところ — In the Middle of Doing