Been practicing grammar a bit and part of that is the polite negative past tense forms. Genki has the verb converted to the ます form, negated, then でした added to the end. However, Bunpro defaults to the casual past negative form じゃなかった with です attached to the end (though with ませんでした in the alternate grammar forms you can cycle through).
Is there a difference between the contexts where these forms would be used? Or are they just two equally valid routes to the same outcome (polite → negative → past, vs negative → past → polite)?
I mean, I don’t think you’re completely off your rocker. I’m crap at explaining any grammar as it comes 90% from immersion. I get what works or not, not how or why mostly. And, explaining it in English is a huge hurdle, ngl.
Important note: I’m neither native nor an expert by any means, so take me with a grain of salt. I’m sure someone’s gonna be along shortly to tell me why every word below is wrong
That doesn’t look quite right to me. ます and です are at the same level of formality in most cases, and you can’t necessarily tack です onto anything - in fact, that’s one of the ways characters in manga and anime can be portrayed as childish, by just tacking です onto everything with no regard for grammar and thinking that makes them polite. I believe (but it’s been a while so I might be wrong) Dekomori from Chuunibyou Demo Koi Ga Shitai is an example of this.
Plain form non-past verbs for instance are never followed with です - 明日は東京に行くです is not grammatically valid, and would instead be 明日は東京に行きます to express that level of politeness.
That said, I think this question might be referring specifically to ではありませんでした vs じゃなかったです, which are semantically equivalent but on slightly different levels of formality - both technically in the same “register” when it comes to politeness (丁寧語) but with contractions generally being at least a little bit informal ではありませんでした would come off a bit more formal than じゃありませんでした, which in turn would come off a bit more formal than じゃなかったです.
That extends to other verbs too, which may also be what the question is referring to instead - 来なかったです is a bit less formal than 来ませんでした.
In general though, you can consider です and ます to be at the same level of politeness, both being 丁寧語.
Interestingly enough with です being acceptable after い-adjectives you see the same thing in present-tense negations: both ～ないです and ～ません are used, though the difference from what I can tell appears to be less in the level of formality but more in ～ないです being a bit less firm. That may extend to the past tense, I’m not sure on that.
I found an interesting stackexchange answer on this topic which references an academic paper. Follow the link for more (including translated extracts from the paper), but the brief summary it gives is:
～ません and ～ないです are semantically equivalent, but ～ないです is softer and less insistent. If you need to give a firm denial with no wiggle room, go with ～ません. But since this level of bluntness can be inappropriate in some situations, ～ないです is there if you need it.
(with the theory being that this is because for ～ません the negation feels a bit more in focus than the politeness, whereas with ～ないです because the です comes last it gives a bit less emphasis to the negation and a bit more to the politeness.)
For spoken Japanese, anyway. In written Japanese ～ません is much more common.
