The endings al and ail change to aux in the plural:
Noun: un cheval (horse)
Masculine singular un cheval
Masculine plural des chevaux
Noun: un travail (task, job)
Masculine singular un travail
Masculine plural des travaux
The endings au, eau, and eu take an X for plural:
Noun: un tuyau (pipe, tip)
Masculine singular un tuyau
Masculine plural des tuyaux
Noun: un château (castle)
Masculine singular un château
Masculine plural des châteaux
Noun: un feu (fire)
Masculine singular un feu
Masculine plural des feux
Notes
- These rules are similar for making adjectives plural
- Compound nouns have their own plural rules.
