Each of these expressions has a distinct meaning of its own.
(a) FEW is a Negative Adjective and signifies NOT MANY.
1. He read few books (he did not read many books).
2. Few women can keep a secret.
(b) A FEW is an Affirmative Adjective and signifies SOME AT LEAST - a certain number & however few.
1. He read a few books. (That is….he read some books at least, though the number was small).
2. He had only a few rupees left.
3. A few days' rest is needed.
(c) THE FEW implies two statements - one Negative and the other Affirmative.
He read the few books he had.
(1) The books he had were not many. (Negative)
(2) He read all the books he had. (Affirmative)
Here a = one - MANY A MAN means many times one man or many men. It is used with a Noun and Verb in the singular, but has in reality a plural meaning. MANY MEN takes the men collectively. MANY A MAN takes them singly.
1. Many a young man has served his country nobly.
Definite Numeral Quantities are sometimes Collective Nouns and as in the case of MANY, the OF is understood after them.
2. A dozen (of) sheep…
3. A million (of) stars…
4. A hundred (of) years…
5. A thousand (of) years…
6. A hundred-thousand rupees…
But we must say A LAKH OF RUPEES and not A LAKH RUPEES.
With a score and a gross the OF must be expressed, but not with two score.