INDEFINITE ARTICLE (A) (Nominative case

un livre (m.), a book, plur.: des livres, books (some) un*appel " a roll-call, " des appels, roll-calls (some) un*hiver " a winter, " des hivers, winters (some) une classe (f.), a class, " des classes, classes (some) une école " a school, " * des écoles, schools (some) une herbe " an herb, " des herbes, herbs (some)

*un (indefinite article) and un (numéral) are pronounced identically, see p. 305 No. 43, as to variations.