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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."
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grammar
How to say regrets [I should not have X]
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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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 **る** んじゃなかった」 = "I should not have eaten."
>
> 「食べ **た** 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 **る** べきではなかった。」
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