Okay, here’s the deal. Generally speaking, you ensure that something will happen, and you insure something when you guarantee it against some kind of loss or harm.
However, as usual, it’s not that simple. Historically, the words insure and ensure have been used interchangeably, and you can throw assure into that crazy mix as well. Even better, the American Heritage Dictionary’s definition of ensure is: “To make sure or certain; insure.” Ah, helpful.
Everyone was apparently fine with this arrangement until — and this is something that may interest our friends over at GrammarBlog — Americans came along and mucked it all up. As the American Heritage Book of English Usage explains, “Although ensure and insure are generally interchangeable, only insure is now widely used in American English in the commercial sense of ‘to guarantee persons or property against risk.'”
As an editor, MRP will continue to differentiate between ensure and insure as I mentioned above. Here’s a helpful way (courtesy of American Heritage) to remember the difference between the three (unlike that dreadful i before e tip I mentioned the other day): I assure you that we have insured the grounds to ensure that we will be protected in case of a lawsuit stemming from an accident.