Most French nouns add -s when pluralised in addition to the plural article, 'les', because the -s is not pronounced. la pomme --> les pommes [the apple --> the apples] le lac --> les lacs [the lake --> the lakes] une banane --> des bananes [a banana --> some bananas] un livre --> trois livres [a book --> three books] Nouns that end in -al change to -aux. le journal --> les journaux [the newspaper --> the newspapers] un cheval --> des chevaux [a horse --> some horses] Nouns that end in -eau change to -eaux. le chapeau --> les chapeaux [the hat --> the hats] un gâteau --> des gâteaux [a cake --> some cakes] un cadeau --> quatre cadeaux [a present/gift --> four presents/gifts] le château --> les châteaux [the castle --> the castles] Nouns that end in -eu change to -eux but I can't think of any examples off the top of my head. It's also worth noting that nouns ending with -s, -x, or -z remain the same in plural form and the only thing that changes is the article. le mois --> les mois [the month --> the months]

"Nouns that end in –au take –x in the plural. For example: bateau (boat) becomes bateaux (boats)" according to dummies.com also some nouns that end in -ou