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.
