Spelling rules: Most common nouns take their plural form by adding an s, un chat: des chats.
Nouns ending in "au", "eau", "eu" take an x in the plural. un cheveu : des cheveux. There are four exceptions: landau, sarrau, pneu and bleu take an s.
Nouns ending in or take an s. un voyou, des voyous. There are seven exceptions: bijou, caillou, genou, pou, joujou, chou and hibou take an x.
Nouns ending in parl are pluralized with aux, un journal des journaux. A few exceptions form their plural with an "s": festival, bal, chacal, carnaval, récital, régal, cérémonial, ...
Nouns ending in ail form their plural with an "s". Exception: aspirail, bail, corail, émail, fermail, soupirail, travail, vantail and vitrail are pluralized with -aux.