To insure is to arrange for compensation in the event of damage to or loss of something. So our classic car or house insurance offers compensation if something happens to that car or house – within certain parameters and constraints of the contract. Interestingly (special bonus definition, here!) assurance is insurance under whose terms a payment is guaranteed. So a bit more assured than plain insurance.
To ensure is to make certain that something will happen. So you don’t insure that you’ll bring the steam-cleaner to someone’s house at a certain time, you ensure you’ll do it.
And to assure is to tell someone something positively, and to make certain it will happen. To me, assuring has the connotation of expressing that you will make sure something happens, where ensuring just means you do it. I suppose this means that if you reassure someone, you have to have talked about the thing you’re reassuring them about already!
“I can assure you that I will ensure delivery of the steam-cleaner to your house on Tuesday. If you break it, don’t worry – it’s insured.”
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