A while back, I received an email that promised to “clarify” the appropriate use of assure, ensure, and insure. It offered more confusion than clarification. So I would like to offer you my own explication:
Assure takes a personal direct object; so you assure a person of something, making him or her confident of its certainty. THAT is it! You never assure that… The personal object may be implied, as in “rest assured,” but it is always there.
The difference between ensure and insure gets a little more fuzzy. Older usage and spellings of these two words were not clearly distinguished, and some still use them interchangeably. But LMI style is based on Chicago style, so we use ensure as the general term meaning to make sure that something will (or won’t) happen, insure is reserved for underwriting financial risk.
To recap: We assure someone, we ensure events, and we insure things.