What this is
In English, we usually add an s to make a noun plural. French does exactly the same thing, with one major twist: the plural s is almost always completely silent. Instead of hearing the noun change, you hear the words around it change—specifically, the articles like le becoming les or un becoming des.
There are also a few families of nouns that do something different instead of adding a simple s, such as changing their ending to x or aux.
The rule
The basic rule for most nouns is to add an -s to the singular form.
However, since this -s is silent, the crucial marker of a plural noun is the article placed before it. When speaking, you must listen to the article to know if the noun is singular or plural.
For example, le chat (the cat) becomes les chats (the cats). Both sound identical in pronunciation for the word chat, but les makes it clear that we are talking about more than one cat.
Why it works this way
French pronunciation evolved faster than its spelling. Hundreds of years ago, those plural s endings were pronounced. Over time, speakers dropped the final consonants in everyday speech to make sentences flow faster, but the written spelling stayed the same. The language compensated by making the preceding articles carry the burden of showing plurality.
Edge cases
Nouns that already end in -s, -x, or -z
If a noun already ends in -s, -x, or -z in its singular form, it does not change at all in the plural. You do not add another letter. For example: le fils (the son) becomes les fils (the sons). Le nez (the nose) becomes les nez. Le prix (the price) becomes les prix.
Nouns ending in -eau and -eu
Nouns that end in -eau or -eu add an -x instead of an -s. The -x is also silent. For example: le bateau becomes les bateaux, and le jeu becomes les jeux.
Nouns ending in -al and -ail
Most nouns ending in -al drop the -al and add -aux. For example: le cheval becomes les chevaux, and l’hôpital becomes les hôpitaux. A limited set of nouns ending in -ail also follow this pattern, such as le travail becoming les travaux.
However, there are common exceptions that just take a standard -s: le festival becomes les festivals, le bal becomes les bals, and le carnaval becomes les carnavals.
Nouns ending in -ou
Most nouns ending in -ou follow the regular rule and add an -s, like le trou becoming les trous and le clou becoming les clous. But there is a small, famous list of exceptions that take an -x instead. The most common ones you will encounter are le bijou (jewel) becoming les bijoux, and le chou (cabbage) becoming les choux.
Irregular plurals: l’œil
A tiny handful of French words completely change their form in the plural. The most prominent one is l’œil (the eye), which becomes les yeux (the eyes). It shares no letters with its singular counterpart!
Liaison reveals the plural
We said the plural -s is silent, and that is mostly true. But if the noun starts with a vowel, the silent -s of the plural article (les, des) wakes up and attaches to the vowel, creating a “z” sound. This is called liaison. For example, les arbres is pronounced “lay-zarbre” and les hommes is pronounced “lay-zomme”.
What English speakers get wrong here
The hardest adjustment for English speakers is letting go of the need to pronounce the plural -s on the noun. We are so used to emphasizing the end of the word to show plurality that it takes conscious effort to shift that emphasis to the beginning (the article). Remember: the article is the hero here. Rely on les and des, and let the end of the noun stay silent.