Dual Adverbs

Recently I discussed my new project for trying to benefit the people I talk to and make them feel more comfortable. One of the ways I've been trying to do this is to find two ways of saying the same thing (like "Have you taken out the trash yet?" and "Have you taken out the trash already?") and look for subtle differences in meaning.

And sometimes I get distracted by patterns.

There are several pairs of words like "always" and "sometimes" that I've been calling dual adverbs, because "not always" means the same thing as "sometimes not":

• "Strangers are not always nice." = "Strangers are sometimes not nice."

"Everywhere" and "somewhere" work the same way, in that "not everywhere" means the same as "somewhere not":

• "It's not messy everywhere." = "Somewhere it's not messy."

(These are reminiscent of the dual quantifiers "for all" and "there exists" from logic: If it's not true that all cars are red, there must exist a car that is not red, so "not for all" = "there exists (such that) not." That's why I'm calling these pairs of adverbs dual.)

Another example with a similar flavor is "totally" and "partially": not totally = partially not.

But here are some examples that surprise me, where it's not so easy to see it as an instance of "for all" versus "there exists":

not often = usually not

• "It's not often raining" = "It usually isn't raining."

not yet = still not

• "It's not ready yet" = "It still isn't ready."

My pattern-collecting self wants to keep on looking for more pairs of dual adverbs, but let me ask you: what do you think are some of the subtle differences between the meanings of these sentences? What does one suggest that the other doesn't? Let me know in the comments!