I. 若A → B为原命题,则B → A为逆命题,~A → ~B为否命题,~B → ~A为逆否命题 (A → B C,逆否命题为~B/~C → ~A),(A B → C,则其逆否命题为~C → ~A/~B)
II. unless = if not,即not A unless B表达为 A → B A unless B表达为非A → B only if:A only if B表达为A → B if only:A if only B表达为B → A A is necessary to B: B → A Whenever,whoever,wherever:= if