Edit Note:
This question has been linked to these questions about using an article before adjectives modifying numbers. This question here has nothing to do with adjectives at all:
- Indefinite articles used with plural nouns: It was AN amazing TWO DAYS
- Indefinite article in the "An [adjective] [number] [plural noun]" construction
The question
Why do we use the article 'a' when referring to 100, but not when referring to any other plural counting?
Here's an example:
I saw a hundred birds today.
But when talking about other count that 100, we don't use 'a' because it's plural:
- I saw ninety-nine birds today.
- I saw two birds today.
- I saw two thousand birds.
- I saw seventy-five birds.
Is there a grammatical or historical explanation?
Best Answer
a is replacing one.
That is, in the following pairings, both options are legitimate:
Preferences for one or the other may vary. To me, using one sounds more precise than a, so that would influence my usage: in the first pairing, "one" sounds better to me, while "a" sounds better in the second, unless I was trying to emphasize that I earned exactly one million dollars.
Regardless, either option is legitimate because a is a synonym for one. The reason your other examples don't work is that they don't start with one.