hi!
masculine singular form = le cahier
masculine plural form = les cahiers
those are determined : determinants definite the '= masculine, feminine and plural= le, la les
for undefined like a/an are in French = un , une, des , it will be a/an
except you don"t have any plural for underfined
le cahier =the exercise book/ des cahiers = the exercises books
un cahier = an exercise book / des cahiers = exercise books
hope I helped