I was taught that "little" is the negative form of "a little" [1] [2] [3]. So why don't we use "little" in these sentences? :
I'm a little confused.
I'm a little worried that the Biden team seems to want to do these things with full pay-fors.
"I'm a little concerned that the intentions aren't genuine if I'm going to be completely honest."
For me, either form has a negative feeling, but I also know that the correct form is "a little" in these sentences. But why?
I don't think it's accurate to say that "little" is the negative, the opposite, of "a little". It's more like a milder form.
"I felt little confusion." I felt some confusion, but not much. This wording implies, not enough to be a real problem.
"I felt a little confusion." I felt some confusion. Probably a middling amount. If I was very confused, I'd say, "I felt a lot of confusion" or "I felt a great deal of confusion."
If I wanted to say that everything was clear and I felt no confusion at all, I'd say, "I felt no confusion". (Or really more likely I'd state it positively, like "Everything was very clear".)
Note that sometimes people use mild wording to be polite. Like if you gave a presentation at a business meeting that was really bad and made no sense at all, someone might think, "That was very confusing. What in the world was he trying to say?" But to be polite they might say, "I'm a little confused. What were you trying to say?"
As Juhas notes, "little" is not often used as an adverb like this in American English. If you say it, it sounds rather formal. You might use it in a scholarly paper but few would use it in casual conversation.
In my variety of English (Western US English), "I'm little confused/worried/concerned" sound incorrect. We do not typically use little as an adverb in these situations. There are some exceptions, such as "he is little known", but in general, we'd only use "a little" to modify verbs like confused, worried, or concerned.
However, this may not be the case in all varieties of English. Macmillan Dictionary says this is American English:
as an adverb: I go there very little. I’m little affected by the cold
https://www.macmillandictionary.com/us/dictionary/american/little_1
In contrast, your second link, the Cambridge Dictionary, does not suggest that little is ever used as an adverb:
A little: adverb
We use a little as an adverb of degree. It is more formal than a bit:
He smiled just a little.
Her hands were shaking a little.
https://dictionary.cambridge.org/us/grammar/british-grammar/little-a-little-few-a-few
What this means is that, in your context, for speakers like me, there is not a choice between "little confused" and "a little confused"; only the latter is correct.
But if you altered the sentences, so that we were using little as an adjective, we'd end up with a situation that mostly conforms to what you've been taught:
I felt a little confusion -> I felt some amount (neutral) of confusion
I felt little confusion -> I did not feel much (negative) confusion
He expressed a little worry -> He expressed some amount (neutral) of worry
He expressed little worry -> He did not express much (negative) worry
As an adverb, "a little" means "some small amount".
As an adverb, "little" means "an insignificant amount" or "a problematically small amount". Which of these it means is determined from the context.
One is not "the negative" or "the opposite" of the other. They are simply different.
I care a little. (= I care somewhat)
I care little. (= I don't care any significant amount)
There's a little food in the fridge. (= there's some food, but not a lot)
There's little food in the fridge. (= there's a problematically small amount of food in the fridge)
So your example sentences mean, "I'm somewhat confused/worried/concerned".
They are neutral, and do not imply that the degree of confusion/worry/concern is insignificant or problematic.
The same sentences with "little" would mean, "I'm not confused/worried/concerned to any significant degree."
Little is a negative idea. Little means not much.
There is little time. (not much time, not enough time)
A little is a positive idea. It means some or a small amount.
There is a little time. (some time, enough time)
We can say a little confused, a little worried and a little concerned (NOT little confused, little worried and little concerned)
('Only a little' has a negative meaning)
