Grammar Info
N2 Lesson 9: 17/19

いつの間(ま)にか
suddenly, unnoticed, without noticing, too soon

Structure
いつのまに + Phrase
Details
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使用域 一般
Information
  • [There is a nuance of being surprised at the change. If there is no feeling of surprise, あっという間に can be used instead]
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しばらく入院(にゅういん)していて、久(ひさ)しぶり外(そと)出(で)ると、いつの間(ま)にか冬(ふゆ)になっていたと気(き)づいた。
After being hospitalized, I went outside for the first time in a while and found out that, without realizing it, winter had come.
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いつの間(ま)にか彼女(かのじょ)はネイティブみたいに話(はな)せるようになった。」
"Without me realizing, her speaking has grown to that of a native."
[Since いつのまにか is not used when change is gradual, the sentence suggests that speaker has not seen her for a long time, and during that time she has become proficient with language]
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あなた子供(こども)から目(め)を離(はな)せばいつのまにか迷子(まいご)になるかもしれない。ご注意(ちゅうい)ください。
If you take your eyes off of your child, they might get lost without you even noticing. Please, be careful.
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[AIAIJ] An Integrated Approach to Intermediate Japanese
Page 75