There are cases when the indefinite article preserves its old original meaning of 'one'.

He had hardly spoken a word.

In such cases we can speak of the numeric meaning of the indefinite article:

An hour in the morning is worth two in the evening.

This meaning is generally found with:

· nouns denoting time, measure and weight:

A week or two passed.

" I'll overtake you in a minute, " said Godfrey.

· in certain expressions of quantity:

a lot of, a couple, a great many, etc.

· in the pattern a…of with possessives, as in:

She’s a colleague of mine.

That’s a friend of Bill’s

· after a negative not:

not a word, not a thought, etc.

· in some set-phrases: one at a time, at a draught

· the numerals hundred, thousand, million and the nouns dozen, score.

My new car cost a thousand pounds.

· in expressions of price, speed, ratio etc.:

Sometimes we can use either a/an or o n e:

We’ll be in Australia for one (or a) year.

Wait here for one (or a) minute, and I’ll be with you.

Using one gives a little more emphasis to the number. However, we use one rather than a/an if we want to emphasize that we are talking about only one person or thing rather than two or more:

Do you want one sandwich or more?

Are you staying just one night?

With nouns in the plural some is used.

Oliver's sobs checked his utterance for some minutes.

Note1: We use one, not a/an in the pattern one…other/ another:

Close one eye, and then the other.

Bees carry pollen from one plant to another.

Note2: We use one with the words day, week, month, year, night, winter, etc. or with specific day or month to say when something happened usually in narration to mean a particular, but unspecified day, evening, winter, etc.:

One summer, the family decided to go to the Crimea.

We can use one day to refer to the future.

One day, you will regret this.

6. Before singular, countable nouns after such and in exclamations after what.

What a lovely day today!

It’s such an interesting idea, isn’t it?

But What pretty girls!

7. Nouns with the indefinite article are used after quite and rather.

It’s quite a long story and not a nice one.

He was rather a curious man to look at.

Sometimes quite and rather can be placed after the indefinite article (especially in AmE)

He is a rather clever man.

It’s a quite important problem.

8. Nouns with the indefinite article follow many) the verb is used in the singular)

Many a true word is spoken in jest.

Many an evening he sat staring at the fire.

9. So, as, too, how, however followed by an adjective precede nouns with the indefinite article:

Youth lasts so short a time.

You have too modest an opinion of yourself.

I can’t miss the chance, however big a risk to run.

How honest a man is he?

10. The indefinite article is also used in various descriptions:

He’s got a long face and a turned up nose.