When to add an 's' to 'vingt' and 'cent'
French numbers have a few spelling rules that can catch English speakers off guard. One of the most common quirks involves the letter s at the end of vingt and cent.
In English, we say "two hundred", never "two hundreds". In French, however, these numbers sometimes take a plural s. You will see this in words like quatre-vingts and deux cents. But this s only appears under specific mathematical and positional conditions.
When you multiply twenty or a hundred, the resulting French number takes a plural s at the end. For example, the number eighty is literally four twenties. Because there is more than one twenty, vingt becomes quatre-vingts.
The same logic applies to the hundreds. Two hundred is deux cents, three hundred is trois cents, and this pattern continues all the way up to neuf cents.
There is a major exception to the plural rule. The s only stays if vingt or cent is the very last word in the number. If another number follows immediately after, the s vanishes.
So, while eighty is quatre-vingts, eighty-one becomes quatre-vingt-un. The same happens with hundreds. You write cinq cents for five hundred, but five hundred and four is cinq cent quatre.
The word for a thousand is mille. It never changes form, meaning it never takes an s itself. Two thousand is simply deux mille.
Because mille counts as a number, it triggers the vanishing s rule. If you say two hundred thousand, the word cents loses its s because mille comes after it. You write deux cent mille.
However, the words million and milliard are considered nouns in French, not numbers. Because they are not numbers, they do not cancel the plural s on cent or vingt. Two hundred million is written with the s intact.
To know if you need an s, remember this rule of thumb. Add an s to vingt and cent when they are multiplied (as in quatre-vingts or trois cents) AND they sit at the very end of the number. If any other number follows them, drop the s.