A Or One?

I don't understand the ruling at the first, because a doesn't mean one in front of many, few.
For your question, either a or one is correct.

Thank you.

Yes, I made a mistake there; it only means 'one' before some numbers.

Anyhow, the question still stands; how can 'a' be used instead of 'one' in such a sentence?

Thank you.

Yes, I made a mistake there; it only means 'one' before some numbers.

Anyhow, the question still stands; how can 'a' be used instead of 'one' in such a sentence?

coloraday Anyhow, the question still stands; how can 'a' be used instead of 'one' in such a sentence?

Because in your sentence "a" is equivalent to "one".

"a year ago" is a time expression similar to "a month ago", "a week ago", "a day ago", etc. In all of them "a" could be replaced with "one".

I'm looking for the reason why it's equivalent to 'one' in these time expressions.

I can't seem to find such usage of 'a' instead of 'one' in any dictionary; the closest OALD gets is 'used instead of one before some numbers'.

I'm looking for the reason why it's equivalent to 'one' in these time expressions.

I can't seem to find such usage of 'a' instead of 'one' in any dictionary; the closest OALD gets is 'used instead of one before some numbers'.

coloradaythe closest OALD gets is 'used instead of one before some numbers'.

Maybe we'll have to write in to the editors of the OALD and ask them to expand their definition.

a / an is used to mean 'one' in front of units of measurement as well, not just certain numbers.

a year ago, one year old, a foot long, a meter away, ...

It's quite difficult to conceive of any of these as definite (with 'the') anyway.

If something happened a day ago, no one is much inclined to ask 'which day?'
If something is a centimeter away, no one is much inclined to ask 'which centimeter?'

For this reason, it seems quite strange to a native speaker to hear such things as

*It happened the day ago.
*It's just the centimeter away.

CJ

coloradayI'm looking for the reason why it's equivalent to 'one' in these time expressions. I can't seem to find such usage of 'a' instead of 'one' in any dictionary; the closest OALD gets is 'used instead of one before some numbers'.

Look at it this way:

We say an apple, a pencil, two apples, three pencils, and so on...

We could also say one apple, two apples, three apples if we wanted to count them.

"a year" acts as a unit of measurement, we are counting "years" from one point in the past up to the present moment:

It happened a/one year ago.

It happened two years ago. -- I'm pretty sure that no one would say "the two years ago", or "the years ago".