〜あいだ（に）

〜あいだ (without に) describes an action that continues THROUGHOUT the whole period: ご飯を食べているあいだ、ずっとテレビを見ていた (the whole time I was eating, I was watching TV nonstop). The second clause usually appears with ずっと and is also a continuing action.

〜あいだに (with に) instead describes something that happens at ONE point within that period: 寝ているあいだに、雨が降った (while I was sleeping, it rained). The second clause is a short, one-time action. This is the key difference between あいだ and あいだに.

What comes before is usually 〜ているあいだ（に） or a noun + の: 夏休みのあいだに (during summer vacation). Unlike 〜ながら (one person doing two things at the same time), 〜あいだに allows the two clauses to have different subjects: 私が寝ているあいだに、母が帰ってきた.

Examples

• 昼(ひる)ご飯(はん)を食(た)べているあいだ、ずっとテレビを見(み)ていました。

  The whole time I was eating lunch, I was watching TV.

• 私(わたし)が寝(ね)ているあいだに、雨(あめ)が降(ふ)ったようです。

  It seems it rained while I was sleeping.

• 夏休(なつやす)みのあいだに、この本(ほん)を読(よ)んでしまいたいです。

  I want to finish reading this book during summer vacation.

Related patterns

• 〜ながら
• 〜とき
• 〜までに