Let's take a look at these two:
Ensure = to make sure that something happens or that something is true.
- We will ensure that you get what you paid for.
- Can you ensure I have access to the database?
- Please ensure that the lights are off.
- They want to ensure this doesn't happen again.
Now, the word "insure" has a different meaning.
Insure = to buy insurance for something. Insurance is an agreement in which you pay a company money and they pay your costs if you are sick, or if you have an accident, injury, etc.
- The house is insured for 100,000 dollars.
(Meaning, you can get up to 100,000 dollars if something happens to it.)
- The painting is insured against theft.
(Meaning, you may be paid if it is stolen.)
- She has insured herself against sickness.
(Meaning, she may be paid if she becomes ill.)
In American English "insure" can be used with the meaning of "ensure."
For example: "I can insure you that he will not leave the house."