You thought ensure and insure meant different things, until one day you read a sentence that said, "I will insure the success of the school prom." and got thoroughly confused about how it was used. The confusion that arises from using ensure vs. insure can be eliminated by understanding their basic meaning and their standard usage in the English language.
To ensure is to do something to guarantee the occurrence of an event or condition. This is taken up as a warranty that something is going to happen which you will ascertain.
E.g.George ensured that there were enough chairs to accommodate two hundred people in the hall for the party.
To insure is to take precaution to deal with a certain event that may occur in the future. This meaning is also applicable in the financial sense when one takes up insurance for health, fire, etc. The use of the term insure is most common in the financial sense.
E.g.As a singer, George insured his voice for $1 million.
The Confusion and How to Resolve It
While these are the basic differences, a lot of people are confused because in North American English, it is acceptable to use the two terms interchangeably.
"Our most recent evidence shows that the distinction between ensure and insure is made more often in British written English than in American written English, and a few commentators hold that insure is more common than ensure in American English." Merriam-Webster's Dictionary of English Usage, 1994
Let's take a look at how the Oxford Advanced Learner's Dictionary (country of origin: United Kingdom) and Merriam-Webster Dictionary (country of origin: United States of America) define the two words.
Oxford Advanced Learner's Dictionary
Ensure to make sure that something happens or is definite
Insure to buy insurance so that you will receive money if your property, car, etc. gets damaged or stolen, or if you get ill/sick or die
Ensure to make sure, certain, or safe : guarantee
Insure : 1. to provide or obtain insurance on or for Insure : 2. to make certain especially by taking necessary measures and precautions
To distinguish when to use one instead of the other, keep the following tips in mind.
When using British English, always follow Oxford Dictionary's definition.
Use insure instead of ensure depending on the context only when using North American English. When in doubt, always use ensure. Use insure only if you are talking about it in the financial context.
To conclude, the use of insure instead of ensure is believed to be archaic. However, its usage as a synonym for ensure is still prevalent in several American publications. It may be used as such if the context is clear and specific to the reader.