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How in the world does this:

晩ご飯を食べるんじゃなかった。

translate as "I should not have eaten breakfast"?
This roughly translates as "It was not that I eat breakfast."

18

First and foremost, 「晩{ばん}ご飯{はん}」 means "dinner", and not "breakfast".

Next, let us study the present-tense form of 「~~んじゃなかった」, which will naturally be 「~~んじゃない」. This might give you a clue.

In colloquial speech,

「Verb or Verb Phrase + んじゃない!」

expresses prohibition. In other words, this is an informal negative imperative.

「ひとりであの公園{こうえん}に行くんじゃない!」= "Don't go to that park alone!"

It is imperative (no pun) that you be familiar with this present-tense usage in order to get a feel for the past tense-usage.

「Verb or Verb Phrase + んじゃなかった

expresses what I might call "retrospective prohibition". In the plain word, it would be "regret".

If the sentence "I should not have eaten dinner." expressed the speaker's regret or repent, then that would be an excellent translation of 「晩ご飯を食べるんじゃなかった」. 

This roughly translate as "It was not that I eat breakfast."

No, it does not, I am afraid.

「食べるんじゃなかった」 surely talks about the past, but the past tense is expressed in the 「なかった」, not in「食べる」.

TL;DR:

「食べんじゃなかった」 = "I should not have eaten."

「食べんじゃない/なかった」 = "It was not that I ate." ⇒ "I did something else."

First sentence says "I ate." and the second, "I didn't eat.", so the difference is huge.

Once again, be reminded that this is all colloquial speech. In more formal speech, one would say;

"I should not have eaten." = 「食べべきではなかった。」

  • Great explanation! Could you please help me by answering this question, I still don't get it, and they are pretty similar questions : japanese.stackexchange.com/questions/40861/… – Dekiru Dec 5 '16 at 4:40
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    Are you aware of any reference that mentions these two usages? I have looked through several grammar texts, like the Dictionaries of Japanese Grammar, Tae Kim's Guide, JGram, even the Daijirin... not a single one of them seems to mention these two things. Why is this? It's not that I doubt you, but I find it baffling that something that's supposed to be common is not mentioned in texts. – Project Book May 27 '18 at 2:46
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たべるんじゃない or basically ~んじゃない roughly means that actually.

んじゃない can be used to give suggestions (should have could have). For example if I wanted to tell you that you shouldn't have posted this kind of question on stack overflow

こんな質問、stack overflowでするんじゃなかった

From Maggie-sensei:

Ex. そうやるんじゃないよ、こうだよ!

= Souyarun janaiyo. Kou dayo!

= Don’t do it that way. Do it this way!

Ex. 風邪ひくからこんなところで寝るんじゃないよ!(tough speech)

= Kaze hikukara konna tokoro de nerun janai yo.

= Don’t sleep here or you will catch cold.

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