[語法] (1)数詞, only などと共に用いる場合や, another, others と対照する場合は通例 one： We have two daughters and one son. / There's only one [^×an] apple left. / One [^×A] teacher stayed but all the others went home. (2)there is 構文の a(n)は「1」という数詞的観念が強い： There is a mistake in her composition. 彼女の作文には誤りが(1つ)ある. 次例と比較： Don't make a mistake. 誤りを犯すな(＝Don't make mistakes.).