Once learned, the difference between the two words should be simple, though even The New Yorker, which takes pride in its unique prescriptive snobbery, confuses insure for ensure. Whereas ensure means to make certain that something will occur or take place, insure strictly means to arrange indemnity for or against something or someone. The mistaken use of in- for en-, although uncommon, can lead to confusing and sometimes humorous implications: I shall ensure your death means I will make certain that you die, but I shall insure your death means I’ll arrange monetary compensation in case you die—the minute difference between a capitalist and a villain.