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The following sentence was translated in the following ways:

I shouldn't have slept with the window open.
I regret (that) I was sleeping with the window open.

But why does ~んじゃなかった make the meaning "I shouldn't have slept" rather than "I hadn't been sleeping"?

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I would've thought it was a rhetorical question as in "Was it not the case that I slept with the window open?" – Flaw Apr 26 '12 at 1:34
@Flaw Wouldn't that be ・・・寝たんじゃない? – ジョン Apr 26 '12 at 2:12
@ジョン I suppose. Which is why it struck me as odd because I already had some form of prejudgement when reading the sentence. – Flaw Apr 26 '12 at 2:31
The meaning is accurate as translated. But I can't answer "why" and I'm having trouble finding it in J-J dictionaries. – Hyperworm Apr 26 '12 at 12:18
up vote 5 down vote accepted

In your example, your ~寝【ね】るんじゃなかった is a shortened form of ~寝【ね】ることではなかった.

~Vことだ is a common command form, and ~Vことではない is used by extension as a form of prohibition. In your example, the following transformations have occurred:

  • ではない → じゃない
  • こと → の → ん

In this case, the speaker is basically scolding herself because she should've known not to sleep with the window open.

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The only part that does not feel very intuitive to me is こと→の. – Flaw Apr 26 '12 at 13:19
@Flaw I would vouch for the こと→の. It's a common contraction. (hoping that contraction is the right word here) – ジョン Apr 26 '12 at 13:44
@ジョン It's easy to see that もの→もん and の→ん. But こと→の seems to be a different thing altogether. – Flaw Apr 26 '12 at 13:46
@Flaw Both こと and の are nominalizers, so it's not exactly fair to call it a "transformation." See japanese.stackexchange.com/q/1395/384 -- note that there is some disagreement on ことだ⇔のだ in the comments of the accepted answer – rintaun Apr 26 '12 at 13:47
Though the commanding ことだ seems to be a different case than what they're discussing. – rintaun Apr 26 '12 at 13:54

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