Why is "I see a few trees" correct but "I see a many trees" not?
Why is "I see a few trees" grammatically correct but "I see a many trees" not?
I notice that "I see few trees" and "I see many trees" are both grammatically correct, since "few" and "many" are both determiners.
"I see a few trees" has two determiners. Why is it grammatical in the case of "few" but not in the case of "many"?
The expressions, a few and its uncountable equivalent a little, are really idioms. Etymonline states that a few was originally meant to convey irony,
Unusual ironic use in quite a few "many" (1854), earlier a good few (1803).
Nowadays, the meaning of a few and a little is closer to "some" rather than two or three, or “hardly any“. To avoid ambiguity, and emphasize the meaning of a small number or quantity, the adverb just can be added.
BBC presenter Kavita Puri, whose father Ravi came with just a few pounds… (source)
The determiner many does not suffer from this ambiguity, many means a large number, placing the indefinite article "a" before many is not idiomatic, it sounds weird and something a learner might say.
…*he came with a many pounds in his pocket… (WRONG)
But we can invert the word order, and make the noun it modifies singular
…he came with many a pound in his pocket… (RIGHT)
Once again, many a is an idiom just like a few and a little, and idioms don't follow the normal rules of grammar or meaning.
many a/an formal + literary
— used with a singular noun to refer to a large number of things or people
It remained a mystery for many a year. [=for many years]
I've been there many a time. [=many times]
Many a tale was told. [=many tales were told]Merriam-Webster Learner's Dictionary
Few means "not many (people or things)." It is used to say that there are not a lot of people or things. A few means "some (people or things)." It is used to say that there are a small number of people or things.
I have a few [=some/a small number of] friends.
I have few friends. [=I do not have many friends]
The difference in meaning is subtle, but usually few puts a little more attention on the negative—that there is not a large number (of people or things). A few puts a little more attention on the positive—that there is a small number (of people or things).
(from MW Learner's Dictionary, Ask the Editor)
The word many can act as a determiner, noun, pronoun and adjective meaning consisting of or amounting to a large number of (persons or things). It does have always a positive sense, and therefore no need for another a before it to make it more positive. Also, when many is used as a determiner, another determiner like a cannot be placed before it.
There is an idiomatic expression many a/an that can be used with a singular noun to refer to a large number of things or people.
Many a man has tried but few men have succeeded.