Can anyone give a grammatical explanation why the words hundred, thousand, million, and trillion are singular after plural numbers?
For example, why can't we say three hundreds or 4 thousands or five millions, etc?
One of my students asked me and all I could say is that it is an exception to the rule of bringing a plural word after a plural number!
This phenomenon - the uninflected plural of numerals - is a remnant of Old English. In Old English, words like hund "hundred" and þusend "thousand" were indeclinable. That is, they did not change form based on case or number.
Today this remains but only in some limited situations - specifically, when the number word is preceded by a determiner (a, one, another number). This is also the case in English's sister language Frisian (you knew English had a sister language, right?), where tûzen, etc., don't take a plural form.
There is not necessarily a logical grammatical justification. In the case of million, usage patterns have shifted.
According to Google Ngram, "two millions" has long been the standard expression. However, starting around 1850, its usage declined while "two million" rose in popularity. Around 1920, "two million" became the new norm, and today "two millions" is rarely used.
In contrast, neither "two thousands" nor "two hundreds" has ever been accepted usage.
If I had to speculate on the reason for the difference, it is probably because million has Latin origins, whereas hundred and thousand have Germanic origins. Therefore, the Old English-based inflection rules have always applied to hundred and thousand, as @MarkBeadles says, but the French-based rules for million have only recently started giving way.
I've always thought that when the words hundred, thousand, million and trillion are used after a specific number, they are part of that number. I know that number fifteen, for example, signifies the plural form of the noun that follows, but there's no need to pluralise the word fifteen. Similarly, it looks logical to me not to pluralise the words that form the number. So two hundred and the like is ruled by this logic. Note that the same thing happens with the (less frequent) words dozen and score.
When the number is not specific and you want to talk about a few hundreds of people, for example, you then pluralise hundred because you aren't naming a specific number. This is the case when the non-specific number is followed by the preposition of. If you say a few hundred people, where no preposition intervenes, then hundred behaves in the same way as when the number is specified. A possible explanation for this could be that it follows the rule that forms two hundred because the construct is the same, while the use of the preposition of changes things.
They don't get an s because that's not what they're called.
300 is called three hundred. That is its name. You could say you are counting the number of hundreds, but you're not really, you're just naming something. We could have ended up calling it thirtyty, or something, but we haven't.
Also you can say 3 hundreds or 2 thousands, when you are counting them.
I have 3 hundreds, 14 fifties, and 12 ones.
The only words you can put in plural are nouns. Cardinal and ordinal numbers aren't nouns. Except for example: the 90s were great
