No, not that kind of discovery, but almost as good.
Music's got me feeling so free...
Man am I old.
Ok, so the discovery is of my 口头禅，a phrase my teach used, and is more properly referred to as a mistake.
I have known for a long time which words I used a lot and overuse and feel are somehow different from how chinese people use them or are simply wrong. Really off things you'll likely get around to correcting some day, but things people seem to just not notice or let go are incredibly hard to fix. I suppose if I did more writing (and actually worked on corrections) that might help, but a lot of things are simply oral patterns which might not necessarily show up in writing. OK! So what is the 'mistake?'
The 只/就 distinction combined with the 只是/就是 distinction and use, is to put it lightly, somewhat _______. Anyway, so one of the questions on the practice test had me choose between 而是与就是 when the first clause was preceeded by a 不是。 Something like: 不是我不想帮你，__我帮不了。Now, usually when an 而是 is required, I can just kind of see it, it feels more direct and doesn't interfere with this 'apology'/'concession' feeling. Example:
不是他很笨，而是老师交的特别不好。Here and in many other cases, while there is a 'concession' effect, it is trumped by the explanatory power and use of 而是. It's not A, but rather B. 不是他作弊，(而)是他姐姐作弊。 你不用处罚他。
I'm sure I've been alerted to my error before (I'm kind of remembering it now, maybe), but now it's more clear than ever.
不是。。。就是 only has one 'correct' usage:
他晚上不是打他孩子，就是打他的妻子。At night if he's not beating his kids then he's beating his wife. (If not A, then B. With the implication that both are done most of the time.)
So how have I been fucking this up so long?
只是 can be used as concessionary particle/(副词） with 不是， but apparently 就是 cannot..
I essentially use 就是 to mean "it's just that" after an "it isn't", now apparently, this isn't grammatically 'cool.'
不是你们不知道怎么对待外国人，就是，就是你们不太懂他们的文化。 （Apparently not good enough for HSK or 'grammar'.) I think 只是 would be a better replacement here than 而是。
But then again, we could be justice after all.