Forming Plurals: Adjectives and Nouns

To form the feminine plural, just add an -s, unless it already
ends in an s, then add nothing.  To form the masculine plural,
just add an -s, except in these cases:  -al becomes -aux
(exceptions: banal - banals, and final - finals); -eau

adds an -x; and if it ends in an x or s already, add nothing.  Just
remember to change the le, la, or l' to les.  

And of course there are more exceptions... some adjectives are
invariable and do not have feminine or plural forms.  Compound adjectives,
such as bleu clair (light blue) and vert foncé (dark green),
adjectives that are also nouns, such as or (gold) and argent (silver),
and the words chic (stylish), bon marché or meilleur
marché (inexpensive) never change.

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This article was used with permission from:
Indo-European Languages