Skyberries and Voidmelons or Voidberries and Skymelons

Still (ha!) being confused by that thing where yet and still are roughly synonyms (massive difference in register notwithstanding) and "not yet" and "not still" are verging on antonyms. ("not begun" vs "already ended"). 

I always have a lot of trouble thinking through yet/still esp when trying to translate stuff. 

I *think* it *might* be  

still X = yet X → "X began at some point in the past and continued to happen until [now]"  (where [now] is either deictic or anaphoric "on wednesday it was still raining")

and then:

not still X → "!(still X)"  [*]    "she is not still learning the guitar"
not yet X → "still !(X)"  [**]   "she is not yet learning the guitar"
still not X → "still !(X)"          "she is still not learning the guitar"
yet not X → "still !(X)"  [***]  "she is yet not learning the guitar"

but my head hurts a bit now. Obv I'm not including non-temporal uses such as "yet" meaning "nevertheless".

I think this is probably same thing as that weird English quirk where "must not" ≈ "may not" but "must" != "may"; the "not" scopes oddly with "must (not X)" vs "(may not) X". But there it's kinda easy to bracket them as above. The "verb not" → !(verb) thing is archaic, but I see how it got there.

But with not-yet the "not" feels like it scopes to an argument it's not adjacent to. I know, idioms gonna idiom non-compositionally, but still. (ha again)

[*]  with implication that X definitely has happened in the past but has now stopped, even if a very literal pedant could pretend that it could include the situation where X has never happened (and hence is continuing not to happen.)

[**] nuance difference ofc; "not yet X" implies very heavily that X is expected to happen at some point; "still not X" doesn't imply it nearly as strongly. But the directionality in time is the same — hasn't happened in the past, might happen in the future.

[***] and sounds dated verging on archaic. "yet not" I think is basically reserved for non-temporal uses ("it was damp yet not raining").