one

numeral [基数词]一；一个；一岁；一点钟

• It's never worked from day one.

pronoun [人称代词]一个人；任何人，人们（表泛指）；那个人；（特指的）那种人

• One needs space to be oneself.

adjective [原级]唯一的；一的

• One mistake and you're toast.