The difference between insure and ensure is subtle, especially when one throws the related word “assure” into the mix. Some people have strict rules about the usage of these two words while other people use these words more or less interchangeably. The important thing to know is that, most of the time, interchangeable use is correct, except in very special circumstances, which we will discuss in a moment. Thanks to the fact that these words sound similar, if you accidentally mix them up in a spoken sentence, someone is unlikely to call you out on the difference between insure and ensure.
Assure, insure, and ensure all come from the same Latin root which means “to make secure.” Assure entered the English language first, in the 14th century, with “ensure” and “insure” following in the 1600s. All of these words have very similar meanings, suggesting a sense of making something certain or guaranteeing something.
The main difference between insure and ensure is that you insure something to be prepared in case something bad happens, while you talk about taking steps to ensure that something will (or won't) happen. For example, you insure your house against water and fire damage, but you put snacks in your car to ensure that you will have snacks if you get stuck in traffic. “Assure” is usually used in reference to putting someone's mind at rest and guaranteeing that something will happen, as in “I assured Sally that the memo would be finished by 3:00.”
Because the difference between insure and ensure is subtle, many people use these words interchangeably, unless they are talking about an insurance policy, an agreement with a company in which the company agrees to pay out a set sum in the event of a problem. You insure houses, cars, and other properties, and you can also purchase health and life insurance to guarantee payouts in the event of injury, sickness, or death. In some nations, people refer to life insurance as life assurance, stressing the idea that a payout will be guaranteed at some point because everyone has to die eventually.
Here's an example of all three words used in a sentence: “Joe assured his wife that he would insure their home to ensure that earthquake damage would be covered.” In this case, the protagonist of our brief story assures someone, putting her mind at rest so that she doesn't have to worry by affirming that something will definitely happen. He claims that he will insure his property against damage from earthquakes, preparing for an unfavorable event, to ensure or be certain that if their home is damaged in an earthquake, they will be able to rebuild or repair it.