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Found this quote in Chuunibyou demo Koi ga Shitai!:

余計なこと教えるんじゃなかった

How my brain tried to process it:

I did not teach unnecessary things

English readability aside, it doesn't make any sense anyways, to which I found the subtitles to be

I shouldn't have taught her all that

which in turn completely baffled me. How does the grammar work out here?

Many thanks.

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