It is paradoxical that three words meaning “to make secure or certain,” to quote The American Heritage Dictionary, create confusion in the minds of writers. Assure, ensure, and insure are kissing cousins, but they are not triplets and should not be used interchangeably regardless of context. In American English – British waters are muddier – it may be simplest to assign this trio three distinct roles:
Assure should be used when eliminating doubt or instilling confidence in someone, as in, “Alicia assured her mother that students at Princeton get enough sleep.”
Ensure should be used when making certain of something, as in, “We must ensure that Princeton Prime is fully understood by those with budgetary responsibilities,” although, at the risk of blurring lines, insure can also be employed in this context.
Insure should be used when guarding someone or something against risk, as in “It is not enough to insure the University’s artistic treasures; they must also be carefully cared for.”
But when purchasing insurance in the United Kingdom, be prepared to encounter a sentence like this, courtesy of The Times: “The man who neglects to assure his life does not jeopardize his own future; he jeopardizes that of his wife and family.” Of this, at least, we can rest assured.