This is one of those mistakes I see most often. The three words, assure, ensure, and insure, are often confused. All three words share an element of “making an outcome sure,” but they cannot be used interchangeably. Here are the unique aspects of each word:
Assure is to promise or say with confidence. It is more about saying than doing.
Example: I assure you that you’ll be warm enough.
Ensure is to do or have what is necessary for success.
Example: These blankets ensure that you’ll be warm enough.
Insure is to cover with an insurance policy.
Example: I will insure my home with additional fire and flood policies.
Using the proper word will allow you to communicate your intention clearly.