Some masculine forms end already in the singular form on s or x. For these words there is no difference between singular and plural.
Examples
un
accident
dangereux
des
accidents
dangereux
an
accident
dangerous
un
homme
serieux
des
hommes
serieux
a
man
serious
un
mauvais
livre
des
mauvais
livres
a
bad
book
* Further, there is a specialty that the plural article (les) is omitted, if the adjective is put before the noun (if de + les = des, then now only de is used).
