The words,  insure, ensure and assure are often used incorrectly.  What is the difference between insure and ensure, and also, assure?  As  www.vocabulary.com succinctly states:

“To assure is to tell someone everything’s ok, to ensure is to make certain, and to insure is to protect financially.”

In fact, all these words – insure, ensure and assure – refer to the act of making someone or something more secure, or sure.
Insure is to be solely used when you want to describe the act of  providing or obtaining insurance to guarantee – belongings, a house – against damage or loss.
Ensure can be used especially ‘to make certain’ of something, and also to guarantee, to follow through to make sure it is completed.
Assure means to promise, to make someone more confident in something, and mostly in the sense of removing mistrust from the person you are assuring.

 insure, ensure and assure – how each is used in a sentence

I assured him that I would not eat any more doughnuts.
The Hurricane Emergency Services ensured that all the inhabitants were evacuated by noon.
I am really pleased that I am insured for collision damage to my car.

There are instances where the words insure, ensure and assure have been used interchangeably, but this is taking liberties with the definitions and is not considered grammatically correct.  If you go to some European countries, you will notice that life insurance is referred to as  “life assurance”.  Such “Life Assurance” companies say their terminology is correct, because by using the word  ‘assurance’ when it comes to Life Insurance, you are in fact meaning that yes, the policy holder or  named beneficiaries, will definitely collect, and therefore you are assuring someone of some money down the line. American companies strictly keep to the use of the word “insurance”even when referring to life insurance.

Although generally referring to the same thing, insure, ensure and assure are not at all interchangeable however, and the definitions of the words help to demonstrate their different meanings.