“for long”

Very curious. I see what you mean. I was about to say it's not *strictly* true; for example, you can ask "Will you be long?" or "Will you be there for long?" But on reflection, "long" is still a negative in both questions, almost as if "long" really means "too long."

So yeah, interesting observation. But I have no clue as to the answer. :p

I guess this is comes from usage, not a "rule". It is like asking, if the past tense of "cheat" is "cheated", why isn't the past tense of "eat" "eated".

I would say it has to do with the rhythm and implication of sound in a sentence. If someone says "I saw her for long", people would understand the meaning, but it would sound as if the speaker's thought was cut off.

It is idiomatic but maybe I can shed some light on it ... or muddle it more.

I won’t be long. » I’ll be long. ... Here, long is an adverb so that is a different thing.

In your byspel, it is a noun meaning a long interval or period of time: see you before long; it will not be for long.

Soooo ... Now we get into a murky way to describe it but maybe it'll help.

For and fore (from before) also work as prefixes with strongly unlike meanings (which often leads to muddling). For, as a prefix, works to intensify and/or give a negative meaning. For byspel: fordone = undone = ruined.

If you think of it as it is used as prefix, and in this byspel, a negative one, then it becomes clearer that it is intensifying the negation of the whole sentence ... almost like a double negative ... I won't be there for long.

So maybe you can see that if you use it in a "positive" sentence then it muddles the meaning ... "I will be there for long" is almost like saying, "I'll be there ... not long" which is gainsaying the meaning of "I will be there for a long time".

Having said all that, it is idiomatic. There's truly no way to foretell when "for" will just be an intensifier or when it will be a negation ... In forever, forgive, and forlorn, it is an intensifier. In forgo (to do without), it is a negation.

I likely just made it worse but I gave it my best shot!

It's about what's implied:- by saying that you didn't see her for long, the implication is that too have seen her for longer would have been preferable.
If you were talking about someone whom spending more than a minute with would be like dying, then it's length itself that is again where the emphasis lies, 'I saw her for the longest time'.

Try putting 'enough' after long to see the effect.

There is a set of words that is usually only used in questions and negatives. For instance "any" and its derivatives.
I didn't see anyone.
Do you want any eggs?
*I saw anyone.
*I want any eggs.

This doesn't really explain it, but I think it may add some thoughts to the ring:

I was there for a while. _c.f._ I was there for long.
I won't be there for a while. _c.f._ I won't be there for long.

In this case while works in both cases. But has quite different meanings. In the first it suggests you were in the place for a long time. In the second it suggests you will not be in that place until a time a while away. Strange hey?

Little mistake there!
Cf. rather than c.f.!

Very interesting. It always struck me as odd that one says "Won't you go?" but one can't say "Will not you go?" In fact one would have to drag it out to "Will you not go?" which sounds downright antique.

A nice question but there are other words which have to have negative connotations: could you be mayed if you saw something gusting? Or dismayed/disgusting?

I have sort of the same question.
Is it:
The door remains open too long
or
The door remains open for too long

Are both correct possibly? Which one do you prefer?